std::string gethex( unsigned int c )//参数必须是int，你懂的
{
	std::ostringstream stm ;
	stm << '%' << std::hex << std::nouppercase << c ;
	return stm.str();
}

std::string encode(std::string str)
{
	static const std::string unreserved = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
		"abcdefghijklmnopqrstuvwxyz"
		"-_.~";

	std::string r;

	for( unsigned int i = 0; i<str.length(); i+=1)
	{
		unsigned char c = str.at(i);//这里必须是无符号型，你懂得
		if( unreserved.find(c) != -1 ) r += c ; else r += gethex(c) ;
	}

	return r;
}

c++之http URL十六进制编码