Dynamic exception specification

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C++
 
C++ language
 
Exceptions
throw-expression
function-try-block
try/catch block
noexcept specifier(C++11)
noexcept operator(C++11)
Dynamic exception specification(until C++20)
 

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

Syntax

throw( ) (1) (deprecated in C++11)(removed in C++20)
throw(typeid, typeid, ...) (2) (deprecated in C++11)(removed in C++17)
1) Non-throwing dynamic exception specification
(until C++17)
1) Same as noexcept(true)
(since C++17)
2) Explicit dynamic exception specification

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 throw()(deprecated) or noexcept, the set is empty.
2) Otherwise, if the declaration of f, pf, or pmf uses a dynamic exception specification(deprecated), the set consists of the types listed in that specification
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, 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, which 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, and
  • the function is named by an id-expression (either directly or as part of member access or pointer-to-member access expression), the set of potential exceptions of the named function is added to the list.
  • if the function is named by an expression of type noexcept function or by an expression of type pointer to noexcept function, 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 polymorphic glvalue, the set consists of std::bad_typeid
6) If e is a new-expression with a non-constant 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) have exception specifications, selected as follows:

  • If the set of potential exceptions is the set of all types, the implicit exception specification is noexcept(false).
  • 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 noexcept(true) and the function type is "noexcept function" 
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);
};
 (since C++17)

Example

Run this code
#include <iostream>
#include <exception>
#include <cstdlib>
 
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();
}

Output: 

That was unexpected

See also

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