将string转换为int并使用stringstream获取C ++中消耗的字符数

Question

I am new to C++ (coming from a C# background) and am trying to learn how to convert a string to an int.

I got it working by using a stringstream and outputting it into a double , like so:

const char* inputIndex = "5+2";
double number = 0;
stringstream ss(inputIndex);
ss >> number;
// number = 5

This works great. The problem I'm having is that the strings I'm parsing start with a number, but may have other, not digit characters after the digits (eg "5+2", "9-(3+2)", etc). The stringstream parses the digits at the beginning and stops when it encounters a non-digit, like I need it to.

The problem comes when I want to know how many characters were used to parse into the number. For example, if I parse 25+2 , I want to know that two characters were used to parse 25 , so that I can advance the string pointer.

So far, I got it working by clearing the stringstream , inputting the parsed number back into it, and reading the length of the resulting string:

ss.str("");
ss << number;

inputIndex += ss.str().length();

While this does work, it seems really hacky to me (though that might just be because I'm coming from something like C#), and I have a feeling that might cause a memory leak because the str() creates a copy of the string.

Is there any other way to do this, or should I stick with what I have?

Thanks.

Answer 1

You can use std::stringstream::tellg() to find out the current get position in the input stream. Store this value in a variable before you extract from the stream. Then get the position again after you extract from the stream. The difference between these two values is the number of characters extracted.

double x = 3435;
std::stringstream ss;
ss << x;

double y;
std::streampos pos = ss.tellg();
ss >> y;
std::cout << (ss.tellg() - pos) << " characters extracted" << std::endl;

Answer 2

The solution above using tellg() will fail on modern compilers (such as gcc-4.6).

The reason for this is that tellg() really shows the position of the cursor, which is now out of scope. See eg " file stream tellg/tellp and gcc-4.6 is this a bug? "

Therefore you need to also test for eof() (meaning the entire input was consumed).