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noexcept specifier (since C++11)

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Specifies whether a function will throw exceptions or not.

Contents

Syntax

noexcept (1)
noexcept(Template:sparam) (2)

Explanation

If the value of the constant Template:sparam is true, the function is declared to not throw any exceptions. noexcept without a constant expression is equivalent to noexcept(Template:sparam).

One of the uses of the constant Template:sparam is (along with the noexcept operator) to define templated functions 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.

If a function marked noexcept allows an uncaught exception to escape at runtime, std::terminate is called immediately.

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().

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

Template:cpp/language/dcl list noexceptTemplate:cpp/language/dcl list except specTemplate:cpp/language/dcl list throw
obtains an rvalue reference if the move constructor does not throw
(function template) [edit]