conversion from empty string to float using stringstream results in non-zero value

Question

I have tried to solve this with previously answered questions like Conversion from string to float changes the number but I have not been successful.

In my code I take a string full of ' ' characters and convert it to float using stringstream. It worked fine (returned me a zero valued float) until I performed another conversion right after that. When a conversion is executed afterwards, the value stored in the float previously converted is not zero, but 4.57048e-41. I hope the following code explains my problem more clearly.

I started with:

std::stringstream ss;
float a;
float b;
for(int i=0; i<LIM; ++i){
    //some other conversions using same stringstream
    //clearing stringstream    
    ss.str( std::string() );
    ss.clear();

    ss << str1;    //string full of empty spaces, length of 5
    ss >> a;
    std::cout << a;//prints zero
}

That worked just fine, but when I changed it to

std::stringstream ss;
float a;
float b;
for(int i=0; i<LIM; ++i){
    //some other conversions using same stringstream
    //clearing stringstream    
    ss.str( std::string() );
    ss.clear();

    ss << str1;    //string full of empty spaces, length of 5
    ss >> a;
    std::cout << a;//prints 4.57048e-41

    ss.str ( std::string() );
    ss.clear();

    ss << str2;    //another string full of empty spaces, length of 5
    ss >> b;
    std::cout << b;//prints zero
}

I am using gcc 4.6.3 with the following flags: -o2 -Wall -Wextra -ansi -pedantic

Any kind of help will be greatly appreciated, but I am not willing to use doubles.

Many thanks!

Answer 1

If the conversion fails, then the target value isn't changed. In your case, it still has its original uninitialised value; so printing it gives garbage or other undefined behaviour.

You should check whether the conversion succeeded:

if (!(ss >> a)) {
    a = 0; // or handle the failure
}

or use conversion functions like std::stof in C++11, or boost::lexical_cast , which throw to indicate conversion failure. (Or, as mentioned in the comments, just set it to zero to start with if you don't otherwise need to detect failure).