noexcept operator

The noexcept operator performs a compile-time check that returns true if an expression is declared to not throw any exceptions.

It can be used within a function template's noexcept specifier to declare that the function will throw exceptions for some types but not others.

Syntax

noexcept( expression )

Returns a prvalue of type bool.

Explanation

The noexcept operator does not evaluate expression.

The result is false if the expression contains at least one of the following potentially evaluated constructs:

call to any type of function that does not have non-throwing exception specification, unless it is a constant expression.
throw expression.
dynamic_cast expression when the target type is a reference type, and conversion needs a run time check
typeid expression when argument type is polymorphic class type

In all other cases the result is true.

(until C++17)

The result is true if the set of potential exceptions of the expression is empty, and false otherwise.

(since C++17)

Keywords

noexcept.

Example

#include <iostream>
#include <utility>
#include <vector>
 
void may_throw();
void no_throw() noexcept;
auto lmay_throw = []{};
auto lno_throw = []() noexcept {};
class T{
public:
  ~T(){} // dtor prevents move ctor
         // copy ctor is noexcept
};
class U{
public:
  ~U(){} // dtor prevents move ctor
         // copy ctor is noexcept(false)
  std::vector<int> v;
};
class V{
public:
  std::vector<int> v;
};
 
int main()
{
 T t;
 U u;
 V v;
 
 std::cout << std::boolalpha
            << "Is may_throw() noexcept? " << noexcept(may_throw()) << '\n'
            << "Is no_throw() noexcept? " << noexcept(no_throw()) << '\n'
            << "Is lmay_throw() noexcept? " << noexcept(lmay_throw()) << '\n'
            << "Is lno_throw() noexcept? " << noexcept(lno_throw()) << '\n'
            << "Is ~T() noexcept? " << noexcept(std::declval<T>().~T()) << '\n'
            // note: the following tests also require that ~T() is noexcept because
            // the expression within noexcept constructs and destroys a temporary
            << "Is T(rvalue T) noexcept? " << noexcept(T(std::declval<T>())) << '\n'
            << "Is T(lvalue T) noexcept? " << noexcept(T(t)) << '\n'
            << "Is U(rvalue U) noexcept? " << noexcept(U(std::declval<U>())) << '\n'
            << "Is U(lvalue U) noexcept? " << noexcept(U(u)) << '\n'  
            << "Is V(rvalue V) noexcept? " << noexcept(V(std::declval<V>())) << '\n'
            << "Is V(lvalue V) noexcept? " << noexcept(V(v)) << '\n';  
}

Output:

Is may_throw() noexcept? false
Is no_throw() noexcept? true
Is lmay_throw() noexcept? false
Is lno_throw() noexcept? true
Is ~T() noexcept? true
Is T(rvalue T) noexcept? true
Is T(lvalue T) noexcept? true
Is U(rvalue U) noexcept? false
Is U(lvalue U) noexcept? false
Is V(rvalue V) noexcept? true
Is V(lvalue V) noexcept? false

noexcept specifier	requires a function to not throw any exceptions (C++11)
exception specification	specifies what exceptions are thrown by a function (deprecated)

noexcept operator

Syntax

Explanation

Keywords

Example

See also