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std.exception

This module defines functions related to exceptions and general error handling. It also defines functions intended to aid in unit testing.
Synopsis of some of std.exception's functions:
string synopsis()
{
    FILE* f = enforce(fopen("some/file"));
    // f is not null from here on
    FILE* g = enforce!WriteException(fopen("some/other/file", "w"));
    // g is not null from here on

    Exception e = collectException(write(g, readln(f)));
    if (e)
    {
        ... an exception occurred...
        ... We have the exception to play around with...
    }

    string msg = collectExceptionMsg(write(g, readln(f)));
    if (msg)
    {
        ... an exception occurred...
        ... We have the message from the exception but not the exception...
    }

    char[] line;
    enforce(readln(f, line));
    return assumeUnique(line);
}
Authors:
Andrei Alexandrescu and Jonathan M Davis
auto assertNotThrown(T : Throwable = Exception, E)(lazy E expression, string msg = null, string file = __FILE__, size_t line = __LINE__);
Asserts that the given expression does not throw the given type of Throwable. If a Throwable of the given type is thrown, it is caught and does not escape assertNotThrown. Rather, an AssertError is thrown. However, any other Throwables will escape.
Parameters:
T The Throwable to test for.
E expression The expression to test.
string msg Optional message to output on test failure. If msg is empty, and the thrown exception has a non-empty msg field, the exception's msg field will be output on test failure.
string file The file where the error occurred. Defaults to __FILE__.
size_t line The line where the error occurred. Defaults to __LINE__.
Throws:
AssertError if the given Throwable is thrown.
Returns:
the result of expression.
Examples:
import core.exception : AssertError;

import std.string;
assertNotThrown!StringException(enforce!StringException(true, "Error!"));

//Exception is the default.
assertNotThrown(enforce!StringException(true, "Error!"));

assert(collectExceptionMsg!AssertError(assertNotThrown!StringException(
           enforce!StringException(false, "Error!"))) ==
       `assertNotThrown failed: StringException was thrown: Error!`);
void assertThrown(T : Throwable = Exception, E)(lazy E expression, string msg = null, string file = __FILE__, size_t line = __LINE__);
Asserts that the given expression throws the given type of Throwable. The Throwable is caught and does not escape assertThrown. However, any other Throwables will escape, and if no Throwable of the given type is thrown, then an AssertError is thrown.
Parameters:
T The Throwable to test for.
E expression The expression to test.
string msg Optional message to output on test failure.
string file The file where the error occurred. Defaults to __FILE__.
size_t line The line where the error occurred. Defaults to __LINE__.
Throws:
AssertError if the given Throwable is not thrown.
Examples:
import core.exception : AssertError;
import std.string;

assertThrown!StringException(enforce!StringException(false, "Error!"));

//Exception is the default.
assertThrown(enforce!StringException(false, "Error!"));

assert(collectExceptionMsg!AssertError(assertThrown!StringException(
           enforce!StringException(true, "Error!"))) ==
       `assertThrown failed: No StringException was thrown.`);
T enforce(E : Throwable = Exception, T)(T value, lazy const(char)[] msg = null, string file = __FILE__, size_t line = __LINE__)
if (is(typeof(() { if (!value) { } } )));
Enforces that the given value is true.
Parameters:
T value The value to test.
E Exception type to throw if the value evalues to false.
const(char)[] msg The error message to put in the exception if it is thrown.
string file The source file of the caller.
size_t line The line number of the caller.
Returns:
value, if cast(bool)value is true. Otherwise, new Exception(msg) is thrown.

Note: enforce is used to throw exceptions and is therefore intended to aid in error handling. It is not intended for verifying the logic of your program. That is what assert is for. Also, do not use enforce inside of contracts (i.e. inside of in and out blocks and invariants), because they will be compiled out when compiling with -release. Use assert in contracts.

Example:

auto f = enforce(fopen("data.txt"));
auto line = readln(f);
enforce(line.length, "Expected a non-empty line.");

T enforce(T, Dg, string file = __FILE__, size_t line = __LINE__)(T value, scope Dg dg)
if (isSomeFunction!Dg && is(typeof(dg())) && is(typeof(() { if (!value) { } } )));
Enforces that the given value is true.
Parameters:
T value The value to test.
Dg dg The delegate to be called if the value evaluates to false.
file The source file of the caller.
line The line number of the caller.
Returns:
value, if cast(bool)value is true. Otherwise, the given delegate is called.

The safety and purity of this function are inferred from Dg's safety and purity.
T enforce(T)(T value, lazy Throwable ex);
Enforces that the given value is true.
Parameters:
T value The value to test.
Throwable ex The exception to throw if the value evaluates to false.
Returns:
value, if cast(bool)value is true. Otherwise, ex is thrown.

Example:

auto f = enforce(fopen("data.txt"));
auto line = readln(f);
enforce(line.length, new IOException); // expect a non-empty line

T errnoEnforce(T, string file = __FILE__, size_t line = __LINE__)(T value, lazy string msg = null);
Enforces that the given value is true, throwing an ErrnoException if it is not.
Parameters:
T value The value to test.
string msg The message to include in the ErrnoException if it is thrown.
Returns:
value, if cast(bool)value is true. Otherwise, new ErrnoException(msg) is thrown. It is assumed that the last operation set errno to an error code corresponding with the failed condition.

Example:

auto f = errnoEnforce(fopen("data.txt"));
auto line = readln(f);
enforce(line.length); // expect a non-empty line

template enforceEx(E : Throwable) if (is(typeof(new E("", "std/exception.d", 612))))

template enforceEx(E : Throwable) if (is(typeof(new E("std/exception.d", 624))) && !is(typeof(new E("", "std/exception.d", 624))))
If !value is false, value is returned. Otherwise, new E(msg, file, line) is thrown. Or if E doesn't take a message and can be constructed with new E(file, line), then new E(file, line) will be thrown.
This is legacy name, it is recommended to use enforce!E instead.

Example:

auto f = enforceEx!FileMissingException(fopen("data.txt"));
auto line = readln(f);
enforceEx!DataCorruptionException(line.length);

T enforceEx(T)(T value, lazy string msg = "", string file = __FILE__, size_t line = __LINE__);
Ditto
T collectException(T = Exception, E)(lazy E expression, ref E result);
Catches and returns the exception thrown from the given expression. If no exception is thrown, then null is returned and result is set to the result of the expression.
Note that while collectException can be used to collect any Throwable and not just Exceptions, it is generally ill-advised to catch anything that is neither an Exception nor a type derived from Exception. So, do not use collectException to collect non-Exceptions unless you're sure that that's what you really want to do.
Parameters:
T The type of exception to catch.
E expression The expression which may throw an exception.
E result The result of the expression if no exception is thrown.
Examples:
int b;
int foo() { throw new Exception("blah"); }
assert(collectException(foo(), b));

int[] a = new int[3];
import core.exception : RangeError;
assert(collectException!RangeError(a[4], b));
T collectException(T : Throwable = Exception, E)(lazy E expression);
Catches and returns the exception thrown from the given expression. If no exception is thrown, then null is returned. E can be void.
Note that while collectException can be used to collect any Throwable and not just Exceptions, it is generally ill-advised to catch anything that is neither an Exception nor a type derived from Exception. So, do not use collectException to collect non-Exceptions unless you're sure that that's what you really want to do.
Parameters:
T The type of exception to catch.
E expression The expression which may throw an exception.
string collectExceptionMsg(T = Exception, E)(lazy E expression);
Catches the exception thrown from the given expression and returns the msg property of that exception. If no exception is thrown, then null is returned. E can be void.
If an exception is thrown but it has an empty message, then emptyExceptionMsg is returned.

Note that while collectExceptionMsg can be used to collect any Throwable and not just Exceptions, it is generally ill-advised to catch anything that is neither an Exception nor a type derived from Exception. So, do not use collectExceptionMsg to collect non-Exceptions unless you're sure that that's what you really want to do.
Parameters:
T The type of exception to catch.
E expression The expression which may throw an exception.
Examples:
void throwFunc() { throw new Exception("My Message."); }
assert(collectExceptionMsg(throwFunc()) == "My Message.");

void nothrowFunc() {}
assert(collectExceptionMsg(nothrowFunc()) is null);

void throwEmptyFunc() { throw new Exception(""); }
assert(collectExceptionMsg(throwEmptyFunc()) == emptyExceptionMsg);
enum string emptyExceptionMsg;
Value that collectExceptionMsg returns when it catches an exception with an empty exception message.
pure nothrow immutable(T)[] assumeUnique(T)(T[] array);

pure nothrow immutable(T)[] assumeUnique(T)(ref T[] array);
Casts a mutable array to an immutable array in an idiomatic manner. Technically, assumeUnique just inserts a cast, but its name documents assumptions on the part of the caller. assumeUnique(arr) should only be called when there are no more active mutable aliases to elements of arr. To strengthen this assumption, assumeUnique(arr) also clears arr before returning. Essentially assumeUnique(arr) indicates commitment from the caller that there is no more mutable access to any of arr's elements (transitively), and that all future accesses will be done through the immutable array returned by assumeUnique.
Typically, assumeUnique is used to return arrays from functions that have allocated and built them.
Parameters:
T[] array The array to cast to immutable.
Returns:
The immutable array.

Example:

string letters()
{
  char[] result = new char['z' - 'a' + 1];
  foreach (i, ref e; result)
  {
    e = cast(char)('a' + i);
  }
  return assumeUnique(result);
}

The use in the example above is correct because result was private to letters and is inaccessible in writing after the function returns. The following example shows an incorrect use of assumeUnique.

Bad:

private char[] buffer;
string letters(char first, char last)
{
  if (first >= last) return null; // fine
  auto sneaky = buffer;
  sneaky.length = last - first + 1;
  foreach (i, ref e; sneaky)
  {
    e = cast(char)('a' + i);
  }
  return assumeUnique(sneaky); // BAD
}

The example above wreaks havoc on client code because it is modifying arrays that callers considered immutable. To obtain an immutable array from the writable array buffer, replace the last line with:
return to!(string)(sneaky); // not that sneaky anymore

The call will duplicate the array appropriately.

Note that checking for uniqueness during compilation is possible in certain cases, especially when a function is marked as a pure function. The following example does not need to call assumeUnique because the compiler can infer the uniqueness of the array in the pure function:
string letters() pure
{
  char[] result = new char['z' - 'a' + 1];
  foreach (i, ref e; result)
  {
    e = cast(char)('a' + i);
  }
  return result;
}

For more on infering uniqueness see the unique and lent keywords in the ArchJava language.

The downside of using assumeUnique's convention-based usage is that at this time there is no formal checking of the correctness of the assumption; on the upside, the idiomatic use of assumeUnique is simple and rare enough to be tolerable.

nothrow T assumeWontThrow(T)(lazy T expr, string msg = null, string file = __FILE__, size_t line = __LINE__);
Wraps a possibly-throwing expression in a nothrow wrapper so that it can be called by a nothrow function.
This wrapper function documents commitment on the part of the caller that the appropriate steps have been taken to avoid whatever conditions may trigger an exception during the evaluation of expr. If it turns out that the expression does throw at runtime, the wrapper will throw an AssertError.

(Note that Throwable objects such as AssertError that do not subclass Exception may be thrown even from nothrow functions, since they are considered to be serious runtime problems that cannot be recovered from.)
Parameters:
T expr The expression asserted not to throw.
string msg The message to include in the AssertError if the assumption turns out to be false.
string file The source file name of the caller.
size_t line The line number of the caller.
Returns:
The value of expr, if any.
Examples:
import std.math : sqrt;

// This function may throw.
int squareRoot(int x)
{
    if (x < 0)
        throw new Exception("Tried to take root of negative number");
    return cast(int)sqrt(cast(double)x);
}

// This function never throws.
int computeLength(int x, int y) nothrow
{
    // Since x*x + y*y is always positive, we can safely assume squareRoot
    // won't throw, and use it to implement this nothrow function. If it
    // does throw (e.g., if x*x + y*y overflows a 32-bit value), then the
    // program will terminate.
    return assumeWontThrow(squareRoot(x*x + y*y));
}

assert(computeLength(3, 4) == 5);
pure nothrow @trusted bool doesPointTo(S, T, Tdummy = void)(auto ref const S source, ref const T target)
if (__traits(isRef, source) || isDynamicArray!S || isPointer!S || is(S == class));

pure nothrow @trusted bool doesPointTo(S, T)(auto ref const shared S source, ref const shared T target);

pure nothrow @trusted bool mayPointTo(S, T, Tdummy = void)(auto ref const S source, ref const T target)
if (__traits(isRef, source) || isDynamicArray!S || isPointer!S || is(S == class));

pure nothrow @trusted bool mayPointTo(S, T)(auto ref const shared S source, ref const shared T target);
Checks whether a given source object contains pointers or references to a given target object.
Parameters:
S source The source object
T target The target object
Returns:
true if source's representation embeds a pointer that points to target's representation or somewhere inside it.

If source is or contains a dynamic array, then, then these functions will check if there is overlap between the dynamic array and target's representation.

If source is a class, then it will be handled as a pointer.

If target is a pointer, a dynamic array or a class, then these functions will only check if source points to target, not what target references.

If source is or contains a union, then there may be either false positives or false negatives:

doesPointTo will return true if it is absolutely certain source points to target. It may produce false negatives, but never false positives. This function should be prefered when trying to validate input data.

mayPointTo will return false if it is absolutely certain source does not point to target. It may produce false positives, but never false negatives. This function should be prefered for defensively choosing a code path.

Note: Evaluating doesPointTo(x, x) checks whether x has internal pointers. This should only be done as an assertive test, as the language is free to assume objects don't have internal pointers (TDPL 7.1.3.5).

Examples:
Pointers
int  i = 0;
int* p = null;
assert(!p.doesPointTo(i));
p = &i;
assert( p.doesPointTo(i));
Examples:
Structs and Unions
struct S
{
    int v;
    int* p;
}
int i;
auto s = S(0, &i);

// structs and unions "own" their members
// pointsTo will answer true if one of the members pointsTo.
assert(!s.doesPointTo(s.v)); //s.v is just v member of s, so not pointed.
assert( s.p.doesPointTo(i)); //i is pointed by s.p.
assert( s  .doesPointTo(i)); //which means i is pointed by s itself.

// Unions will behave exactly the same. Points to will check each "member"
// individually, even if they share the same memory
Examples:
Arrays (dynamic and static)
int i;
int[]  slice = [0, 1, 2, 3, 4];
int[5] arr   = [0, 1, 2, 3, 4];
int*[]  slicep = [&i];
int*[1] arrp   = [&i];

// A slice points to all of its members:
assert( slice.doesPointTo(slice[3]));
assert(!slice[0 .. 2].doesPointTo(slice[3])); // Object 3 is outside of the
                                              // slice [0 .. 2]

// Note that a slice will not take into account what its members point to.
assert( slicep[0].doesPointTo(i));
assert(!slicep   .doesPointTo(i));

// static arrays are objects that own their members, just like structs:
assert(!arr.doesPointTo(arr[0])); // arr[0] is just a member of arr, so not
                                  // pointed.
assert( arrp[0].doesPointTo(i));  // i is pointed by arrp[0].
assert( arrp   .doesPointTo(i));  // which means i is pointed by arrp
                                  // itself.

// Notice the difference between static and dynamic arrays:
assert(!arr  .doesPointTo(arr[0]));
assert( arr[].doesPointTo(arr[0]));
assert( arrp  .doesPointTo(i));
assert(!arrp[].doesPointTo(i));
Examples:
Classes
class C
{
    this(int* p){this.p = p;}
    int* p;
}
int i;
C a = new C(&i);
C b = a;

// Classes are a bit particular, as they are treated like simple pointers
// to a class payload.
assert( a.p.doesPointTo(i)); // a.p points to i.
assert(!a  .doesPointTo(i)); // Yet a itself does not point i.

//To check the class payload itself, iterate on its members:
()
{
    foreach (index, _; Fields!C)
        if (doesPointTo(a.tupleof[index], i))
            return;
    assert(0);
}();

// To check if a class points a specific payload, a direct memmory check
// can be done:
auto aLoc = cast(ubyte[__traits(classInstanceSize, C)]*) a;
assert(b.doesPointTo(*aLoc)); // b points to where a is pointing
class ErrnoException: object.Exception;
Thrown if errors that set errno occur.
final @property uint errno();
Operating system error code.
CommonType!(T1, T2) ifThrown(E : Throwable = Exception, T1, T2)(lazy scope T1 expression, lazy scope T2 errorHandler);

CommonType!(T1, T2) ifThrown(E : Throwable, T1, T2)(lazy scope T1 expression, scope T2 delegate(E) errorHandler);

CommonType!(T1, T2) ifThrown(T1, T2)(lazy scope T1 expression, scope T2 delegate(Exception) errorHandler);
ML-style functional exception handling. Runs the supplied expression and returns its result. If the expression throws a Throwable, runs the supplied error handler instead and return its result. The error handler's type must be the same as the expression's type.
Parameters:
E The type of Throwables to catch. Defaults to Exception
T1 The type of the expression.
T2 The return type of the error handler.
T1 expression The expression to run and return its result.
T2 errorHandler The handler to run if the expression throwed.
Returns:
expression, if it does not throw. Otherwise, returns the result of errorHandler.

Example:

//Revert to a default value upon an error:
assert("x".to!int().ifThrown(0) == 0);

You can also chain multiple calls to ifThrown, each capturing errors from the entire preceding expression.

Example:

//Chaining multiple calls to ifThrown to attempt multiple things in a row:
string s="true";
assert(s.to!int().
        ifThrown(cast(int)s.to!double()).
        ifThrown(cast(int)s.to!bool())
        == 1);

//Respond differently to different types of errors
assert(enforce("x".to!int() < 1).to!string()
        .ifThrown!ConvException("not a number")
        .ifThrown!Exception("number too small")
        == "not a number");

The expression and the errorHandler must have a common type they can both be implicitly casted to, and that type will be the type of the compound expression.

Example:

//null and new Object have a common type(Object).
static assert(is(typeof(null.ifThrown(new Object())) == Object));
static assert(is(typeof((new Object()).ifThrown(null)) == Object));

//1 and new Object do not have a common type.
static assert(!__traits(compiles, 1.ifThrown(new Object())));
static assert(!__traits(compiles, (new Object()).ifThrown(1)));

If you need to use the actual thrown exception, you can use a delegate.

Example:

//Use a lambda to get the thrown object.
assert("%s".format().ifThrown!Exception(e => e.classinfo.name) == "std.format.FormatException");

enum RangePrimitive: int;
This enum is used to select the primitives of the range to handle by the handle range wrapper. The values of the enum can be OR'd to select multiple primitives to be handled.
RangePrimitive.access is a shortcut for the access primitives; front, back and opIndex.

RangePrimitive.pop is a shortcut for the mutating primitives; popFront and popBack.
front

back

popFront

popBack

empty

save

length

opDollar

opIndex

opSlice

access

pop

auto handle(E : Throwable, RangePrimitive primitivesToHandle, alias handler, Range)(Range input)
if (isInputRange!Range);
Handle exceptions thrown from range primitives.
Use the RangePrimitive enum to specify which primitives to handle. Multiple range primitives can be handled at once by using the OR operator or the pseudo-primitives RangePrimitive.access and RangePrimitive.pop. All handled primitives must have return types or values compatible with the user-supplied handler.
Parameters:
E The type of Throwable to handle.
primitivesToHandle Set of range primitives to handle.
handler The callable that is called when a handled primitive throws a Throwable of type E. The handler must accept arguments of the form E, ref IRange and its return value is used as the primitive's return value whenever E is thrown. For opIndex, the handler can optionally recieve a third argument; the index that caused the exception.
Range input The range to handle.
Returns:
A wrapper struct that preserves the range interface of input.

opSlice: Infinite ranges with slicing support must return an instance of std.range.Take when sliced with a specific lower and upper bound (see std.range.primitives.hasSlicing); handle deals with this by takeing 0 from the return value of the handler function and returning that when an exception is caught.

Examples:
import std.algorithm : equal, map, splitter;
import std.conv : to, ConvException;

auto s = "12,1337z32,54,2,7,9,1z,6,8";

// The next line composition will throw when iterated
// as some elements of the input do not convert to integer
auto r = s.splitter(',').map!(a => to!int(a));

// Substitute 0 for cases of ConvException
auto h = r.handle!(ConvException, RangePrimitive.front, (e, r) => 0);
assert(h.equal([12, 0, 54, 2, 7, 9, 0, 6, 8]));
Examples:
import std.algorithm : equal;
import std.range : retro;
import std.utf : UTFException;

auto str = "hello\xFFworld"; // 0xFF is an invalid UTF-8 code unit

auto handled = str.handle!(UTFException, RangePrimitive.access,
        (e, r) => ' '); // Replace invalid code points with spaces

assert(handled.equal("hello world")); // `front` is handled,
assert(handled.retro.equal("dlrow olleh")); // as well as `back`