How do you construct a std::string with an embedded null?

If I want to construct a std::string with a line like:

std::string my_string("a\0b");

Where i want to have three characters in the resulting string (a, null, b), I only get one. What is the proper syntax?

Since C++14

we have been able to create literal std::string

#include <iostream>
#include <string>

int main()
{
    using namespace std::string_literals;

    std::string s = "pl-\0-op"s;    // <- Notice the "s" at the end
                                    // This is a std::string literal not
                                    // a C-String literal.
    std::cout << s << "\n";
}

Before C++14

The problem is the std::string constructor that takes a const char* assumes the input is a C-string. C-strings are \0 terminated and thus parsing stops when it reaches the \0 character.

To compensate for this, you need to use the constructor that builds the string from a char array (not a C-String). This takes two parameters - a pointer to the array and a length:

std::string   x("pq\0rs");   // Two characters because input assumed to be C-String
std::string   x("pq\0rs",5); // 5 Characters as the input is now a char array with 5 characters.

Note: C++ std::string is NOT \0-terminated (as suggested in other posts). However, you can extract a pointer to an internal buffer that contains a C-String with the method c_str().

Also check out Doug T's answer below about using a vector<char>.

Also check out RiaD for a C++14 solution.

If you are doing manipulation like you would with a c-style string (array of chars) consider using

std::vector<char>

You have more freedom to treat it like an array in the same manner you would treat a c-string. You can use copy() to copy into a string:

std::vector<char> vec(100)
strncpy(&vec[0], "blah blah blah", 100);
std::string vecAsStr( vec.begin(), vec.end());

and you can use it in many of the same places you can use c-strings

printf("%s" &vec[0])
vec[10] = '\0';
vec[11] = 'b';

Naturally, however, you suffer from the same problems as c-strings. You may forget your null terminal or write past the allocated space.

I have no idea why you'd want to do such a thing, but try this:

std::string my_string("a\0b", 3);

What new capabilities do user-defined literals add to C++? presents an elegant answer: Define

std::string operator "" _s(const char* str, size_t n) 
{ 
    return std::string(str, n); 
}

then you can create your string this way:

std::string my_string("a\0b"_s);

or even so:

auto my_string = "a\0b"_s;

There's an "old style" way:

#define S(s) s, sizeof s - 1 // trailing NUL does not belong to the string

then you can define

std::string my_string(S("a\0b"));