Understanding const reference in assignment when constructing object-CodePudding

Today I see this piece of code and I'm wondering to know what it is exactly doing this const reference in an assignment where a new object is created. (I don't know how to name this kind of assignments.)

std::string const& p = s.c_str(); // s is a std::string

I understand that something like std::string const& p = s; will create a reference p to s, but in the line shown we are creating a new object (using the raw pointer from std::string::c_str).

I've made a MCVE in Coliru with this:

#include <iostream>
#include <string>

void foo(std::string const& s)
{
    std::string const& p = s.c_str(); // << here
    
    std::cout << s << " " << p << " " << &s << " " << &p << std::endl;
}

int main()
{
    foo("hello");
}

And, as expected the output is showing that a new object was created:

hello hello 0x7ffdd54ef9a0 0x7ffdd54ef950

So, my question is: Is this actually doing something I'm not able to see? Does it have any problem (like a memory leak) in the code?

CodePudding user response：

In fact this construction

std::string const& p = s.c_str();

does not differ essentially from

std::string const p( s.c_str() );

but is more confusing.

As for the output of this statement

std::cout << s << " " << p << " " << &s << " " << &p << std::endl;

then there are outputted two different addresses. The first one is the address of the parameter s and the second one is the address of the temporary object of the type std::string created in this declaration

std::string const& p = s.c_str();

by means of the conversion constructor

basic_string(const charT* s, const Allocator& a = Allocator());

CodePudding user response：

From std::string::c_str's documentation, it returns:

a pointer to an array that contains a null-terminated sequence of characters (i.e., a C-string) representing the current value of the string object. That is, a const char*.

So when you wrote:

std::string const& p = s.c_str();

In the above statement, the const char* that was returned on the right hand side is used to create a temporary object of type std::string using a converting constructor that takes const char* as argument.

Next, the lvalue reference p on the left hand side is bound to that temporary obect. And in doing so, the lifetime of the temporary is extended.