Converting a string to float using std::istringsteam in C++

Question

I've been trying to convert a simple string to a float, but I'm having no luck with it. this is what I've got at the moment:

int main()
{
    float value;
    std::string stringNum = "0.5";
    std::istringstream(stringNum) >> value;

    return 0
}

but I'm getting this error:

Error   2   error C2440: '<function-style-cast>' : cannot convert from 'std::string' to 'std::istringstream'    c:\users\administrator\desktop\Test\main.cpp    12

can anyone give me some guidance here on how to just simply convert the string to a float?

Thanks

Shorter version would be std::istringstream{std::string{"0.5"}} >> value ; — David G
– David G, Commented Jun 13, 2013 at 14:19
@0x499602D2: Even shorter: std::istringstream{"0.5"} >> value; — Andy Prowl
– Andy Prowl, Commented Jun 13, 2013 at 14:21

Andy Prowl · Accepted Answer · 2013-06-13 14:20:12Z

5

Most likely you haven't included all the relevant headers:

#include <string>
#include <sstream>

Here is a live example showing that your code compiles when the appropriate headers are included.

In general, you should not rely on indirect inclusion of a necessary standard header file from another standard header file (unless, of course, this inclusion is documented in the Standard itself).

Also notice, that you are creating a temporary string stream, which will be destroyed at then end of the evaluation of the expression

std::istringstream(stringNum) >> value

You may want to create a stream object this way instead:

std::istringstream ss(stringNum);
ss >> value;

// Here you can use ss again...

edited Jun 13, 2013 at 14:20

answered Jun 13, 2013 at 14:14

Andy Prowl

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