string str = "123";
string 转 int
int i = atoi( str.c_str() );
string 转 float
float f = atof( str.c_str() );
string 转 char
char c [32];
strintf( c, "%s", str.c_str() );
char float int 转 string
string str1;
strintf( c, "%d%.2f", i, f );
str1 += c;