c++ - Filling a std string with memcpy does not set length -

i have following code fill std::string curl response.

struct curl_string {     std::string tmpstr;     size_t len; };  void init_string(struct curl_string *s) {     s->len = 0; }  size_t writefunc(void *ptr, size_t size, size_t nmemb, struct curl_string *s) {     size_t new_len = s->len + size*nmemb;     s->tmpstr.reserve(new_len + 1);      memcpy(&s->tmpstr[0] + s->len, ptr, size*nmemb);     s->tmpstr[new_len] = '\0';     s->len = new_len;      return size*nmemb; } 

the problem is, though reserve correct length, when try read string, size still 0.

i wanted avoid using loop add 1 character @ time. there way still have correct string length in std::string ?

your problem std::string::reserve reserves memory characters, doesn't alter length of string. size isn't recalculated when underlying buffer modified, because near-impossible detect in cases.

the solution use resize instead of reserve:

s->tmpstr.resize(new_len + 1); 

is there reason keeping track of length manually? can call s->tmpstr.size().