noexcept operator (since C++11)

 

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noexcept specifier (C++11)

noexcept operator (C++11)

throw specifier (until C++17*)

 

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. The result is true if the set of potential exceptions of the expression is empty(until C++17)expression is non-throwing(since C++17), and false otherwise.

expression is an unevaluated operand.

If expression is a prvalue, temporary materialization is formally applied. Particularly, the destructor is required to be non-deleted and accessible if such expression is of a class type or (possibly multidimensional) array thereof.

(since C++17)

Keywords

noexcept

Notes

Evaluation of a non-throwing expression is allowed to throw an exception if the evaluation has undefined behavior.

Example

Run this code

#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 <<
        "may_throw() is noexcept(" << noexcept(may_throw()) << ")\n"
        "no_throw() is noexcept(" << noexcept(no_throw()) << ")\n"
        "lmay_throw() is noexcept(" << noexcept(lmay_throw()) << ")\n"
        "lno_throw() is noexcept(" << noexcept(lno_throw()) << ")\n"
        "~T() is 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
        "T(rvalue T) is noexcept(" << noexcept(T(std::declval<T>())) << ")\n"
        "T(lvalue T) is noexcept(" << noexcept(T(t)) << ")\n"
        "U(rvalue U) is noexcept(" << noexcept(U(std::declval<U>())) << ")\n"
        "U(lvalue U) is noexcept(" << noexcept(U(u)) << ")\n"
        "V(rvalue V) is noexcept(" << noexcept(V(std::declval<V>())) << ")\n"
        "V(lvalue V) is noexcept(" << noexcept(V(v)) << ")\n";
}

Output:

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

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
CWG 2722	C++17	it was unclear whether temporary materialization is applied if expression is a prvalue	it is applied in this case

See also

noexcept specifier(C++11)	specifies whether a function could throw exceptions
Dynamic exception specification(until C++17)	specifies what exceptions are thrown by a function (deprecated in C++11)