CODEWITHSUNDEEP
c++
codesavesworld
codesavesworld

Reputation: 793

initialize a class without copy construct

I'm reading a chapter in c++ priemr about copy construct,the book says “During copy initialization, the compiler is permitted (but not obligated) to skip the copy/move constructor and create the object directly. That is, the compiler is permitted to rewrite

string null_book="xxxxxxx";

into

string null_book("xxxxxxx");

even if the compiler omits the call to the copy/move constructor, the copy/move constructor must exist and must be accessible"

then i write a class and delete copy constructor

class myclass {
public:
myclass(int a):id(a){
    std::cout << "construct: "<<id<<std::endl;
}
myclass(const myclass& s) = delete; 
myclass& operator=(const myclass& s) = delete;
private:
int id=0 ;
};

if i do this it compiled successfully

 myclass c = 2;

but this gives me error

myclass c =myclass(2);

i'm totally confused, please help me,thanks a alot

Upvotes: 1

Views: 592

Answers (2)

eerorika
eerorika

Reputation: 238341

myclass c = expr;

is copy initialisation. Before C++17, for this to be well-formed, the type must be copyable or at least movable.

Since C++17, the expression that is used to initialise the temporary (in first case, there is implicit conversion from int that produces the temporary) is used to initialise c instead, so there is no copy/move involved and the program is well-formed in the latest standard.

Upvotes: 1

yumetodo
yumetodo

Reputation: 1202

before C++17

Move constructor is required.

Since C++17

C++ compiler must do RVO(Return Value Optimization) so that move constructor is now not required.

P0135R1 Wording for guaranteed copy elision through simplified value categories

Upvotes: 1

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