带双引号的字符串初始化

Question

I'm doing some tutorial, and had this line

const std::string spaces(greeting.size(), " ");

However that didn't work until i changed double quotes to single quotes like

const std::string spaces(greeting.size(), ' ');

What is the point about quotes in string initialization? I've tried to check http://www.cplusplus.com/reference/string/string/ , but didn't find something that would explain this to me

Answer 1

Double-quoted " " is string literal in C++ (represented by const char [N] ), while single-quoted ' ' is a character literal ( char ), so you were calling (std::size_t, const char [N]) constructor.

It doesn't exist, so you get a compilation error.

You need to change it to one of the available constructors like:

string (size_t n, char c);
string (const char* s);

(which you already did by changing second parameter to ' ' )

Answer 2

That's because you're using this constructor: string (size_t n, char c) (no. 6 in the reference you linked), which fills the newly constructed string with n chars c . If you use double quotes you create a string literal, single quotes mean chars.

Answer 3

There are character literals and string literals in C++.

For example a character enclosed in single quotes like 'A' denotes a character literal and represents one character and has type char .

If you will enclose symbol A in double quotes like "A" then you get a string literal that has type cont char[2] and has internal representation like

{ 'A', '\0' }

that is it consists from two characters including the terminating zero.

You can see the difference running this simple porgram

#includde <iostream>

int main()
{
    std::cout << "sizeof( 'A' ) = " << sizeof( 'A' ) << std::endl;
    std::cout << "sizeof( \"A\" ) = " << sizeof( "A" ) << std::endl;
}

Class std::basic_string (or std::string) has the following constructor

basic_string(size_type n, charT c, const Allocator& a = Allocator());

and in this declaration

const std::string spaces(greeting.size(), ' ');

this constructor is used. It initializes the string with the greeting.size() number of space character.