noexcept specifier (since C++11)

Specifies whether a function will throw exceptions or not.

Syntax

Explanation

void f() noexcept; // the function f() does not throw
void (*fp)() noexcept(false); // fp points to a function that may throw
void g(void pfa() noexcept);  // g takes a pointer to function that doesn't throw
// typedef int (*pf)() noexcept; // error

If any declaration of a function has a noexcept specification that isn't noexcept(false), other declarations of the same function have to have the same noexcept-specification. Similar restrictions apply to overriders of a virtual function and to initialization and assignment of pointers to functions (target entity has to have identical or more permissive exception specification).

void f() noexcept;
void f(); // error, incompatible exception specifications
void g() noexcept(false);
void g(); // ok

If a search for a matching exception handler leaves a function marked noexcept or noexcept(true), std::terminate is called immediately.

Inheriting constructors and the implicitly-declared default constructors, copy constructors, move constructors, destructors, copy-assignment operators, move-assignment operators are all noexcept(true) by default, unless they are required to call a function that is noexcept(false), in which case these functions are noexcept(false).

Any user-defined destructor is noexcept(true) by default, unless the declaration specifies otherwise, or the destructor of any base or member is noexcept(false).

Any deallocation function is noexcept(true) by default, unless the declaration specifies otherwise.

template<class T> T f() noexcept(sizeof(T) < 4);
int main() {
    decltype(f<void>()) *p; // f unevaluated, but noexcept-spec is needed
}

Notes

One of the uses of the constant expression is (along with the noexcept operator) to define function templates that declare noexcept for some types but not others.

Note that a noexcept specification on a function is not a compile-time check; it is merely a method for a programmer to inform the compiler whether or not a function should throw exceptions. The compiler can use this information to enable certain optimizations on non-throwing functions as well as enable the noexcept operator, which can check at compile time if a particular expression is declared to throw any exceptions. For example, containers such as std::vector will move their elements if the elements' move constructor is noexcept, and copy otherwise (unless the copy constructor is not accessible, but a potentially throwing move constructor is, in which case the strong exception guarantee is waived).

Deprecates

noexcept is an improved version of throw(), which is deprecated in C++11. Unlike throw(), noexcept will not call std::unexpected and may or may not unwind the stack, which potentially allows the compiler to implement noexcept without the runtime overhead of throw().

Defect reports

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

Keywords

noexcept.

Example

// whether foo is declared noexcept depends on if the expression
// T() will throw any exceptions
template <class T>
  void foo() noexcept(noexcept(T())) {}
 
void bar() noexcept(true) {}
void baz() noexcept { throw 42; }  // noexcept is the same as noexcept(true)
 
int main() 
{
    foo<int>();  // noexcept(noexcept(int())) => noexcept(true), so this is fine
 
    bar();  // fine
    baz();  // compiles, but at runtime this calls std::terminate
}

See also