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

C++11 actually has functions that do this now, in your case std::stof

Note that as far as handling your validation, it will throw an std::invalid_argument exception if the argument cannot be converted.

For completeness, here are more of such functions

std::stoi    // string to int
std::stol    // string to long
std::stoll   // string to long long
std::stof    // string to float
std::stod    // string to double
std::stold   // string to long double

Answer 2

You do it by not ignoring the second parameter - it will tell you where the scanning stopped. If it's the end of the string then there wasn't an error.

char *ending;
float converted_value = strtof(str_val.c_str(), &ending);
if (*ending != 0) // error

Answer 3

Do neither and use stringstream.

std::stringstream s(str_val);

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