C++ - Uniform initializer with std::string

问题

I am trying the uniform intializer with the string class of C++. Below is the code:

#include <iostream>
#include <string>

using namespace std;

int main()
{
    string str1 {"aaaaa"};
    string str2 {5, 'a'};
    string str3 (5, 'a');

    cout << "str1: " << str1 << endl;
    cout << "str2: " << str2 << endl;
    cout << "str3: " << str3 << endl;

    return 0;
}

The output would be:

str1: aaaaa
str2: a
str3: aaaaa

This made me scratched my head. Why str2 cannot achieved the desired result as str3?

回答1:

std::string has a constructor that takes an initializer_list argument.

basic_string( std::initializer_list<CharT> init,
              const Allocator& alloc = Allocator() );

That constructor always gets precedence when you use a braced-init-list to construct std::string. The other constructors are only considered if the elements in the braced-init-list are not convertible to the type of elements in the initializer_list. This is mentioned in [over.match.list]/1.

Initially, the candidate functions are the initializer-list constructors ([dcl.init.list]) of the class T and the argument list consists of the initializer list as a single argument.

In your example, the first argument 5 is implicitly convertible to char, so the initializer_list constructor is viable, and it gets chosen.

This is evident if you print each character in the strings as ints

void print(char const *prefix, string& s)
{
    cout << prefix << s << ", size " << s.size() << ": ";
    for(int c : s) cout << c << ' ';
    cout << '\n';
}

string str1 {"aaaaa"};
string str2 {5, 'a'};
string str3 (5, 'a');

print("str1: ", str1);
print("str2: ", str2);
print("str3: ", str3);

Output:

str1: aaaaa, size 5: 97 97 97 97 97 
str2: a, size 2: 5 97 
str3: aaaaa, size 5: 97 97 97 97 97

Live demo

回答2:

You are using std::string's constructor with this siganture

std::basic_string( std::initializer_list<CharT> init, const Allocator& alloc = Allocator() );

And the compiler treated 5 as a char type, which translates to an ASCII type that isn't printed on screen. If you change that 5 to a printable value, say A whose ASCII value is 65,