Can I tell if a std::string represents a number using stringstream?

Question

Apparently this is suposed to work in showing if a string is numerical, for example "12.5" == yes, "abc" == no. However I get a no reguardless of the input.

std::stringstream ss("2");
double d; ss >> d;
if(ss.good()) {std::cout<<"number"<<std::endl;}
else {std::cout<<"other"<<std::endl;}

Answer 1

Don't use good()! Test if the stream is failed or not:

if (ss)

Good tells you if any of eofbit, badbit, or failbit are set, while fail() tells you about badbit and failbit. You almost never care about eofbit unless you already know the stream is failed, so you almost never want to use good.

Note that testing the stream directly, as above, is exactly equivalent to:

if (!ss.fail())

Conversely, !ss is equivalent to ss.fail().

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Combining the extraction into the conditional expression:

if (ss >> d) {/*...*/}

Is exactly equivalent to:

ss >> d;
if (ss) {/*...*/}

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However, you probably want to test if the complete string can be converted to a double, which is a bit more involved. Use boost::lexical_cast which already handles all of the cases.