std :: stringstream operator >>无法将字符串转换为float

Question

I can't understand why the second >> fails. Am I doing something wrong or missing some code?

std::ifstream file;
std::stringstream ss;
std::string str;
float f1, f2;

file.open("file.txt");
getline(file, str);
ss.str(str);
ss >> f1;

getline(file, str);//when packed inside if(), evalueates to true
ss.str(str);
ss >> f2; //when packed inside if(), evalueates to false - but why it fails?


std::cout<<"str = "<<str<<"\n";
std::cout<<"ss.str() = "<<ss.str()<<"\n";
std::cout<<"f1 = "<<f1<<"\nf2 = "<<f2<<"\n";

file:

0.120000
0.120000

output:

str = 0.120000
ss.str() = 0.120000
f1 = 0.12
f2 = 2.06831e+032

I have tried this code on multiple files and apparently only 1st insertion into float works, files have an empty line at the end

Edit

as Dan pointed, I tried extracting floats directly from file:

file.open("file.txt");
file >> f1;
file >> f2;

works ideally; also simplyfies code a lot

Answer 1

You have to add the following statement before the second attempt to read:

ss.clear(); 

Why ?

Because when you've read the first line, the string stream contains "0.120000" and ss>>f1 will cause ss to reach the end of file. So that the eof flag is set.

When you reset the stringstreams's content with str() , you don't reset the state flags, so that the attempt to read will fail. adding the ss.clear() after you've reset the stringstreams's content will correct this situation.

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