Hilman Hilman - 1 year ago 86
C++ Question

C++ - Uniform initializer with std::string

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
cannot achieved the desired result as

Answer Source

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);


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

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