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Difference between revisions of "cpp/language/noexcept"

From cppreference.com
< cpp‎ | language
(Added explanation and examples about the C++17 change for non-throwing lambdas)
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As a consequence, the evaluation of simple {{rlp|lambda|lambdas}} without {{rlp|except_spec|exception specification}}, whose body potentially does not throw exceptions, changed in C++17. All of the followings were evaluated as {{ttb|false}} before C++17, but {{ttb|true}} thereafter:
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{{example
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| code=
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#include <iostream>
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auto simple_1 = []{};
 +
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auto simple_2 = []{ constexpr auto c = 3.5; return 1+c; };
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 +
auto simple_3 = [](int a) {
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        if( a>0 ) return simple_2();
 +
   
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        ++a;
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        return a+3.14;
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    };
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int i;
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auto simple_4 = []{ return &i; };
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int main()
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{
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    std::cout << std::boolalpha
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          << "Is simple_1() noexcept? " << noexcept(simple_1()) << '\n'
 +
          << "Is simple_2() noexcept? " << noexcept(simple_2()) << '\n'
 +
          << "Is simple_3() noexcept? " << noexcept(simple_3(1)) << '\n'
 +
          << "Is simple_4() noexcept? " << noexcept(simple_4()) << '\n';
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}
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|output=
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Is simple_1() noexcept? true
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Is simple_2() noexcept? true
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Is simple_3() noexcept? true
 +
Is simple_4() noexcept? true
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}}
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===Keywords===
 
===Keywords===
  

Revision as of 08:47, 25 June 2018

 
 
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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.

Contents

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:

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)

As a consequence, the evaluation of simple lambdas without exception specification, whose body potentially does not throw exceptions, changed in C++17. All of the followings were evaluated as false before C++17, but true thereafter:

#include <iostream>
 
auto simple_1 = []{};
 
auto simple_2 = []{ constexpr auto c = 3.5; return 1+c; };
 
auto simple_3 = [](int a) {
        if( a>0 ) return simple_2();
 
        ++a;
        return a+3.14;
    };
 
int i;
auto simple_4 = []{ return &i; };
 
int main()
{
 
    std::cout << std::boolalpha
           << "Is simple_1() noexcept? " << noexcept(simple_1()) << '\n'
           << "Is simple_2() noexcept? " << noexcept(simple_2()) << '\n'
           << "Is simple_3() noexcept? " << noexcept(simple_3(1)) << '\n'
           << "Is simple_4() noexcept? " << noexcept(simple_4()) << '\n';
}

Output:

Is simple_1() noexcept? true
Is simple_2() noexcept? true
Is simple_3() noexcept? true
Is simple_4() noexcept? true

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

See also

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