Difference between revisions of "cpp/language/explicit"
From cppreference.com
(Corrects copy and paste error for the B direct-list-initialization example.) |
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{{cpp/language/classes/navbar}} | {{cpp/language/classes/navbar}} | ||
| − | Specifies constructors | + | Specifies constructors {{rev inl|since=c++11| and conversion operators}} that don't allow [[cpp/language/implicit_cast|implicit conversions]] or [[cpp/language/copy_initialization|copy-initialization]]. |
===Syntax=== | ===Syntax=== | ||
Revision as of 06:25, 4 May 2015
Specifies constructors and conversion operators(since C++11) that don't allow implicit conversions or copy-initialization.
Syntax
| explicit class_name ( params ) | (1) | ||||||||
| explicit operator type ( ) (since C++11) | (2) | ||||||||
1) specifies that this constructor is only considered for direct initialization (including explicit conversions)
2) specifies that this user-defined conversion function is only considered for direct initialization (including explicit conversions)
Notes
A constructor with a single non-default parameter(until C++11) that is declared without the function specifier explicit is called a converting constructor.
An explicit default constructor can be used to perform both default initialization and value initialization: the use of explicit with a default constructor has no effect unless it has default arguments that would allow it to be used in copy-initialization context.
Both constructors (other than copy/move) and user-defined conversion functions may be function templates; the meaning of explicit doesn't change.
Example
Run this code
struct A { A(int) {} // converting constructor A(int, int) {} // converting constructor (C++11) operator int() const { return 0; } }; struct B { explicit B(int) {} explicit B(int, int) {} explicit operator int() const { return 0; } }; int main() { A a1 = 1; // OK: copy-initialization selects A::A(int) A a2(2); // OK: direct-initialization selects A::A(int) A a3 {4,5}; // OK: direct-list-initialization selects A::A(int, int) A a4 = {4,5}; // OK: copy-list-initialization selects A::A(int, int) int na1 = a1; // OK: copy-initialization selects A::operator int() int na2 = static_cast<int>(a1); // OK: static_cast performs direct-initialization A a5 = (A)1; // OK: explicit cast performs static_cast // B b1 = 1; // Error: copy-initialization does not consider B::B(int) B b2(2); // OK: direct-initialization selects B::B(int) B b3 {4,5}; // OK: direct-list-initialization selects B::B(int, int) // B b4 = {4,5}; // Error: copy-list-initialization does not consider B::B(int,int) // int nb1 = b2; // Error: copy-initialization does not consider B::operator int() int nb2 = static_cast<int>(b2); // OK: static_cast performs direct-initialization B b5 = (B)1; // OK: explicit cast performs static_cast }
