How do I read an entire file into a std::string in C++?

Question

How do I read a file into a std::string, i.e., read the whole file at once?

Text or binary mode should be specified by the caller. The solution should b

Answer 1

If you have C++17 (std::filesystem), there is also this way (which gets the file's size through std::filesystem::file_size instead of seekg and tellg):

#include 
#include 
#include 

namespace fs = std::filesystem;

std::string readFile(fs::path path)
{
    // Open the stream to 'lock' the file.
    std::ifstream f(path, std::ios::in | std::ios::binary);

    // Obtain the size of the file.
    const auto sz = fs::file_size(path);

    // Create a buffer.
    std::string result(sz, '\0');

    // Read the whole file into the buffer.
    f.read(result.data(), sz);

    return result;
}

Note: you may need to use and std::experimental::filesystem if your standard library doesn't yet fully support C++17. You might also need to replace result.data() with &result[0] if it doesn't support non-const std::basic_string data.