Difference between revisions of "cpp/language/except spec"

Revision as of 22:36, 13 December 2021

Lists the exceptions that a function might directly or indirectly throw.

Syntax

An explicit dynamic exception specification shall appear only on a function declarator for a function type, pointer to function type, reference to function type, or pointer to member function type that is the top-level type of a declaration or definition, or on such a type appearing as a parameter or return type in a function declarator.

void f() throw(int);            // OK: function declaration
void (*pf)() throw (int);       // OK: pointer to function declaration
void g(void pfa() throw(int));  // OK: pointer to function parameter declaration
typedef int (*pf)() throw(int); // Error: typedef declaration

Explanation

If a function is declared with type T listed in its dynamic exception specification, the function may throw exceptions of that type or a type derived from it. A dynamic exception speficification without a type-id list (i.e. throw()) is non-throwing. A function with a non-throwing dynamic exception specification does not allow any exceptions.

Incomplete types, pointers or references to incomplete types other than cv void*, and rvalue reference types are not allowed in the exception specification. Array and function types, if used, are adjusted to corresponding pointer types. parameter packs are allowed(since C++11).

A dynamic exception specification is not considered part of a function’s type.

If the function throws an exception of the type not listed in its exception specification, the function std::unexpected is called. The default function calls std::terminate, but it may be replaced by a user-provided function (via std::set_unexpected) which may call std::terminate or throw an exception. If the exception thrown from std::unexpected is accepted by the exception specification, stack unwinding continues as usual. If it isn't, but std::bad_exception is allowed by the exception specification, std::bad_exception is thrown. Otherwise, std::terminate is called.

Potential exceptions

Each function f, pointer to function pf, and pointer to member function pmf has a set of potential exceptions, which consists of types that might be thrown. Set of all types indicates that any exception may be thrown. This set is defined as follows:

Note: for implicitly-declared special member functions (constructors, assignment operators, and destructors) and for the inheriting constructors(since C++11), the set of potential exceptions is a combination of the sets of the potential exceptions of everything they would call: constructors/assignment operators/destructors of non-variant non-static data members, direct bases, and, where appropriate, virtual bases (including default argument expressions, as always)

Each expression e has a set of potential exceptions. The set is empty if e is a core constant expression, otherwise, it is the union of the sets of potential exceptions of all immediate subexpressions of e (including default argument expressions), combined with another set that depends on the form of e, as follows:

void f() throw(int); // f()'s set is "int"
void g();            // g()'s set is the set of all types
 
struct A { A(); };                  // "new A"'s set is the set of all types
struct B { B() noexcept; };         // "B()"'s set is empty
struct D() { D() throw (double); }; // new D's set is the set of all types

All implicitly-declared member functions and inheriting constructors (since C++11)have exception specifications, selected as follows:

• If the set of potential exceptions is the set of all types, the implicit exception specification allows all exceptions (the exception specification is considered present, even though it is inexpressible in code and behaves as if there is no exception specification)(until C++11)is noexcept(false)(since C++11).
• Otherwise, If the set of potential exceptions is not empty, the implicit exception specification lists every type from the set.
• Otherwise, the implicit exception specification is throw()(until C++11)noexcept(true)(since C++11).

struct A
{
    A(int = (A(5), 0)) noexcept;
    A(const A&) throw();
    A(A&&) throw();
    ~A() throw(X);
};
 
struct B
{
    B() throw();
    B(const B&) = default; // exception specification is "noexcept(true)"
    B(B&&, int = (throw Y(), 0)) noexcept;
    ~B() throw(Y);
};
 
int n = 7;
struct D : public A, public B
{
    int * p = new (std::nothrow) int[n];
    // D has the following implicitly-declared members:
    // D::D() throw(X, std::bad_array_new_length);
    // D::D(const D&) noexcept(true);
    // D::D(D&&) throw(Y);
    // D::~D() throw(X, Y);
};

Example

#include <iostream>
#include <exception>
#include <cstdlib>
 
static_assert(__cplusplus < 201700, 
    "ISO C++17 does not allow dynamic exception specifications.");
 
class X {};
class Y {};
class Z : public X {};
class W {};
 
void f() throw(X, Y) 
{
    int n = 0;
    if (n) throw X(); // OK
    if (n) throw Z(); // also OK
    throw W(); // will call std::unexpected()
}
 
int main()
{
    std::set_unexpected([]
    {
        std::cout << "That was unexpected!" << std::endl; // flush needed
        std::abort();
    });
    f();
}

That was unexpected!

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also