Convert float to std::string in C++

Question

I have a float value that needs to be put into a std::string. How do I convert from float to string?

float val = 2.5;
std::string my_val = val;          


        

Answer 1

Use std::to_chars once your standard library provides it:

std::array buf;
auto result = std::to_chars(buf.data(), buf.data() + buf.size(), val);
if (result.ec == std::errc()) {
  auto str = std::string(buf.data(), result.ptr - buf.data());
  // use the string
} else {
  // handle the error
}

The advantages of this method are:

• It is locale-independent, preventing bugs when writing data into formats such as JSON that require '.' as a decimal point
• It provides shortest decimal representation with round trip guarantees
• It is potentially more efficient than other standard methods because it doesn't use the locale and doesn't require allocation

Unfortunately std::to_string is of limited utility with floating point because it uses the fixed representation, rounding small values to zero and producing long strings for large values, e.g.

auto s1 = std::to_string(1e+40);
// s1 == 10000000000000000303786028427003666890752.000000

auto s2 = std::to_string(1e-40);
// s2 == 0.000000

C++20 might get a more convenient std::format API with the same benefits as std::to_chars if the P0645 standards proposal gets approved.