converting string to float in c++

Question

What is the best way to convert a string to float(in c++), given that the string may be invalid. Here are the type of input

20.1

0.07

x

0

Answer 1

Do neither and use stringstream.

std::stringstream s(str_val);

float f;
if (s >> f) {
    // conversion ok
} else {
    // conversion not ok
}