noexcept operator (since C++11)

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
throw-expression try block	handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype (C++11) auto (C++11)	constexpr (C++11) consteval (C++20) constinit (C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization (C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11)   final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Expressions

General
Value categories Order of evaluation Constant expressions Primary expressions	Lambda expressions (C++11) Requires expressions (C++20) Pack indexing expression (C++26) Potentially-evaluated expressions
Literals
Integer literals Floating-point literals Boolean literals Character literals	Escape sequences String literals Null pointer literal (C++11) User-defined literal (C++11)
Operators
Assignment operators Increment and decrement Arithmetic operators Logical operators Comparison operators Member access operators Other operators new-expression delete-expression throw-expression	alignof sizeof sizeof... (C++11) typeid noexcept (C++11) Fold expressions (C++17) Alternative representations of operators Precedence and associativity Operator overloading Default comparisons (C++20)
Conversions
Implicit conversions Explicit conversions Usual arithmetic conversions     User-defined conversion	const_cast static_cast dynamic_cast reinterpret_cast

 

Exceptions

try block

Throwing exceptions

Handling exceptions

Exception specification

noexcept specification (C++11)

dynamic specification (until C++17*)

noexcept operator (C++11)

 

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 specified to be non-throwing(since C++17), and false otherwise.

expression is an unevaluated operand.

If expression is a prvalue, temporary materialization is applied.

(since C++17)

Notes

Even if noexcept(expr) is true, an evaluation of expr may still throw as the result of encountering undefined behavior.

If expression is of a class type or (possibly multidimensional) array thereof, temporary materialization requires the destructor be non-deleted and accessible.

(since C++17)

Keywords

noexcept

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
CWG 2792	C++11	the noexcept operator was required to determine whether exceptions may be thrown in the case of encountering undefined behavior	not required

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)