为什么std :: string.find（）不能用于“<”？

Question

I have :

#include <iostream>
#include <string>

int main(int argc, const char * argv[])
{

    std::string foo = "<!";

    if (foo.find("<")) {
        std::cout<<"Found";
    }

    return 0;
}

Why won't this code find the "<" ? If I change the line to foo.find("!") , then it is found. So what is the problem with "<" ?

Answer 1

If successful std::string::find() returns the index where its argument is found, not a boolean.

Because "<" is found at index 0 it returns 0, and in a boolean context 0 is false, so the condition fails. Because "!" is at index 1 it returns 1, and in a boolean context that is true.

Instead of testing whether find returns true you need to use:

if (foo.find("<") != std::string::npos) {

npos is the special value reserved to mean "not a position" and find returns it to say the value was not found.

NB when you want to search for a single character it is better to do exactly that, not search for a string of length one, ie use foo.find('<') instead of foo.find("<")

You could have debugged this for yourself by trying to search for '<' in the string "!<" and you would have found that the result is nothing to do with the specific character you search for, but only where in the string it gets found.

Answer 2

Your if statement is not checking if you found your substring correctly, try this:

if (foo.find("<") != std::string::npos)

working example

Answer 3

It does find the "<", at position 0, which becomes false in this context.

find returns the index at which it found the argument, or std::string::npos if it can't find the argument. You do not check for that in your if statement.