返回间接类型对象会影响性能吗？

Question

I have an inline function

string myFunction() { return ""; }

Compared with

string myFunction() { return string(); }

does it has a performance sacrifice?

Tested it on VC2012 release with std::string and QString (although on QString, the two returns different results : thanks to DaoWen). Both show the second version is about 3 times faster than the first. Thanks to all your answers and comments. Tested code is attached below

#include <iostream>
#include <string>
#include <ctime>
using namespace std;

inline string fa() { return ""; }
inline string fb() { return string(); }

int main()
{
    int N = 500000000;
    {
        clock_t begin = clock();
        for (int i = 0; i < N; ++i)
            fa();
        clock_t end = clock();
        double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;
        cout << "fa: " << elapsed_secs << "s \n";
    }

    {
        clock_t begin = clock();
        for (int i = 0; i < N; ++i)
            fb();
        clock_t end = clock();
        double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;
        cout << "fb: " << elapsed_secs << "s \n";
    }

    return 0;
}

Answer 1

They will each cause different std::string constructors.

The std::string() -- will create an empty object.

The "" will construct using std::string(char*)

The later will internally do a strlen+strcpy which is not needed in the first, so very small difference.

Answer 2

In the example you posted, return ""; will be automatically translated to return string(""); at compile time . Unless string("") is significantly slower than string() , then there shouldn't be much of a difference.

---

In your comment you mention that you're actually using QString , not std::string . Note that QString() constructs a null string ( isNull() and isEmpty() both return true), whereas QString("") constructs an empty string ( isEmpty() returns true but isNull() returns false). They're not the same thing! You can think of QString() like char * str = NULL; , and QString("") like char * str = ""; .

Answer 3

Use return string(); as that will use the default constructor. A good Standard Library implementation will probably not even initialise the string buffer at that point.

The constructor from const char* must take a string copy. So I think return ""; will be slower.

But, to be really sure, race your horses .