C++: what is the optimal way to convert a double to a string?

Question

What is the most optimal way to achieve the same as this?

void foo(double floatValue, char* stringResult)
{
    sprintf(stringResult, \"%f\", floatV         


        

Answer 1

I'd say sprintf is pretty much the optimal way. You may prefer snprintf over it, but it doesn't have much to do with performance.

Answer 2

If you use the Qt4 frame work you could go :

double d = 5.5;
QString num = QString::number(d);

Answer 3

In the future, you can use std::to_chars to write code like https://godbolt.org/z/cEO4Sd . Unfortunately, only VS2017 and VS2019 support part of this functionality...

#include <iostream>
#include <charconv>
#include <system_error>
#include <string_view>
#include <array>

int main()
{
    std::array<char, 10> chars;
    auto [parsed, error] = std::to_chars(
        chars.data(), 
        chars.data() + chars.size(), 
        static_cast<double>(12345.234)
    );
    std::cout << std::string_view(chars.data(), parsed - chars.data());
}

For a lengthy discussion on MSVC details, see https://www.reddit.com/r/cpp/comments/a2mpaj/how_to_use_the_newest_c_string_conversion/eazo82q/

Answer 4

http://www.cplusplus.com/reference/iostream/stringstream/

double d=123.456;
stringstream s;
s << d; // insert d into s

Answer 5

I'd probably go with what you suggested in your question, since there's no built-in ftoa() function and sprintf gives you control over the format. A google search for "ftoa asm" yields some possibly useful results, but I'm not sure you want to go that far.

Answer 6

The best thing to do would be to build a simple templatized function to convert any streamable type into a string. Here's the way I do it:

#include <sstream>
#include <string>

template <typename T>
const std::string to_string(const T& data)
{
   std::ostringstream conv;
   conv << data;
   return conv.str();
}

If you want a const char* representation, simply substitute conv.str().c_str() in the above.