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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 = enforceEx!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);
}

License:
Boost License 1.0

Authors:
Andrei Alexandrescu and Jonathan M Davis

Source:
std/exception.d

void 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.

Examples:
assertNotThrown!StringException(enforceEx!StringException(true, "Error!"));

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

assert(collectExceptionMsg!AssertError(assertNotThrown!StringException(
           enforceEx!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:
assertThrown!StringException(enforceEx!StringException(false, "Error!"));

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

assert(collectExceptionMsg!AssertError(assertThrown!StringException(
           enforceEx!StringException(true, "Error!"))) ==
       `assertThrown failed: No StringException was thrown.`);

T enforce(T)(T value, lazy const(char)[] msg = null, string file = __FILE__, size_t line = __LINE__) if (is(typeof(() { if (!value) { } } )));
If !!value is true, value is returned. 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, string file, size_t line = __LINE__)(T value, lazy const(char)[] msg = null) if (is(typeof(() { if (!value) { } } )));
Scheduled for deprecation in January 2013. If passing the file or line number explicitly, please use the version of enforce which takes them as function arguments. Taking them as template arguments causes unnecessary template bloat.

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) { } } )));
If !!value is true, value is returned. Otherwise, the given delegate is called.

The whole safety and purity are inferred from Dg's safety and purity.

T enforce(T)(T value, lazy Throwable ex);
If !!value is true, value is returned. 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);
If !!value is true, value is returned. Otherwise, new ErrnoException(msg) is thrown. ErrnoException assumes that the last operation set errno to an error code.

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

template enforceEx(E) if (is(typeof(new E("", "std/exception.d", 524))))
If !!value is true, 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.

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

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);

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 strenghten 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.

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

The use in the example above is correct because result was private to letters and is unaccessible 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 = '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.

Checking for uniqueness during compilation is possible in certain cases (see the unique and lent keywords in the ArchJava language), but complicates the language considerably. The downside of 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.)

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 pointsTo(S, T, Tdummy = void)(auto ref const S source, ref const T target) if (__traits(isRef, source) || isDynamicArray!S || isPointer!S || is(S == class));
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 pointsTo will check if there is overlap between the dynamic array and target's representation.

If source is or contains a union, then every member of the union is checked for embedded pointers. This may lead to false positives, depending on which should be considered the "active" member of the union.

If source is a class, then pointsTo will handle it as a pointer.

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

Note:
Evaluating pointsTo(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).

class ErrnoException: object.Exception;
Thrown if errors that set errno occur.

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.

Examples:
//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.

Examples:
//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 expection, 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");