What is most appropriate in const std::string assignment/declaration? Using the constructor(e.g., const std::string WORD("hello");) or using equal operator(e.g., const std::string WORD= "hello";)?
Does these things has difference in memory usage or time processes?
3 Answers
For any reasonable compiler, the code generated will be the same in both cases. Whether you should use direct initialization or copy-initialization in this case is essentially opinion-based.
Comments
In both cases, generally compilers will remove the copies using "Return Value Optimisation" anyway. See this code on ideone here calls neither the ordinary constructor nor the assignment operator as it doesn't print that they're being called to screen:
That is:
#include <iostream>
class C
{
public:
C() {}
C(const char[]) { std::cout << "Ordinary Constructor" << std::endl; }
C& operator=(const char[]) { std::cout << "Assignment Operator" << std::endl; return *this; }
};
int main() {
std::cout << "Start" << std::endl;
C x1 = "Hello";
C x2("Hello");
std::cout << "End" << std::endl;
}
Simply outputs:
Start
End
It doesn't output:
Start
Assignment Operator
Ordinary Constructor
End
As C++ allows the copies to be skipped and the temporary to be constructed in place.
2 Comments
kraskevich
It would never call assignment operator here because both x1 and x2 are initialized, they are not assigned.
M.M
C x1 = "Hello"; means copy constructor, not assignment operatorThe lines:
std::string x = "hello";
std::string x("hello");
both will only call the constructor of std::string. That is, they are identical, and neither will use the operator= overloads.
operator=used in your shown code.