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

Revision as of 10:53, 13 August 2020

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

Syntax

This specification may appear only on lambda-declarator or on a function declarator that is the top-level(until C++17) declarator of a function, variable, or non-static data member, whose type is a function type, a pointer to function type, a reference to function type, a pointer to member function type. It may appear on the declarator of a parameter or on the declarator of a return type.

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 exception specification, the function may throw exceptions of that type or a type derived from it.

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).

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:

1. If the declaration of f, pf, or pmf uses a dynamic exception specification that does not allow all exceptions(until C++11), the set consists of the types listed in that specification.
2. (since C++11) Otherwise, if the declaration of f, pf, or pmf uses noexcept(true), the set is empty.
3. Otherwise, the set is the set of all types.

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:

1. If e is a function call expression, let g denote the function, function pointer, or pointer to member function that is that is called, then
   • if the declaration of g uses a dynamic exception specification, the set of potential exceptions of g is added to the set;
   • (since C++11) if the declaration of g uses noexcept(true), the set is empty;
   • otherwise, the set is the set of all types.
2. If e calls a function implicitly (it's an operator expression and the operator is overloaded, it is a new-expression and the allocation function is overloaded, or it is a full expression and the destructor of a temporary is called), then the set is the set of that function.
3. If e is a throw-expression, the set is the exception that would be initialized by its operand, or the set of all types for the re-throwing throw-expression (with no operand).
4. If e is a dynamic_cast to a reference to a polymorphic type, the set consists of std::bad_cast.
5. If e is a typeid applied to a dereferenced pointer to a polymorphic type, the set consists of std::bad_typeid.
6. (since C++11) If e is a new-expression with a non-constant array size, the set consists of std::bad_array_new_length.

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