# Dynamic exception specification

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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

- `noexcept` specifier

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