std :: string的奇怪行为

Question

I have this puny program.

#include    <iostream>
#include    <string>
int main()
{
    std::string st = ('='+"10");
    std::cout<<st<<"-"<<st.c_str();
    return 0;    
}

What sort of output you expect without running it?

I am getting : -

I am running into such problems while using boost::spirit library and passing its output around as c-strings.

Am I missing something? I am using gcc 4.6.1 (ubuntu 10.10).

Answer 1

This:

'=' + "10"

Probably does not do what you expect. Rather than concatenating, it will "add" (arithmetically) the "ASCII" value of '=' to a pointer to the literal string "10", which is a buffer overrun and so invokes undefined behavior.

If you run your program under valgrind you will likely see it complain about this.

Instead, try:

std::string st = "=";
st += "10";

Answer 2

Try instead:

#include  <iostream>
#include  <string>
int main()
{
    std::string st = ('='+std::string("10"));
    std::cout<<st<<"-"<<st.c_str();
    return 0;
}

Note "10" is a const char* (pointer). Adding to it will increment the pointer using standard integer arithmetic and not concatenate a string.