In general, it's easiest to parse such typical config files in two stages: first read the lines, and then parse those one by one.
In C++, lines can be read from a stream using std::getline(). While by default it will read up to the next '\n' (which it will consume, but not return), you can pass it some other delimiter, too, which makes it a good candidate for reading up-to-some-char, like = in your example.
For simplicity, the following presumes that the = are not surrounded by whitespace. If you want to allow whitespaces at these positions, you will have to strategically place is >> std::ws before reading the value and remove trailing whitespaces from the keys. However, IMO the little added flexibility in the syntax is not worth the hassle for a config file reader.
const char config[] = "url=http://example.com\n"
"file=main.exe\n"
"true=0";
std::istringstream is_file(config);
std::string line;
while( std::getline(is_file, line) )
{
std::istringstream is_line(line);
std::string key;
if( std::getline(is_line, key, '=') )
{
std::string value;
if( std::getline(is_line, value) )
store_line(key, value);
}
}
(Adding error handling is left as an exercise to the reader.)