c++ - Why does a single pointer break my code? -

i working on making own string, need copy array elements free store allocation char* pointer it. far, constructor takes array of chars size , set sz it. can see, have char* elem private class variable. adding simple declaration, "char* elem" breaks of code. if comment line of code out, program execute , output 5. on otherhand, if leave char* elem pointer output random , erroneous. tried setting char* elem point null later make point newly allocated array, same thing. adding destructor "delete[] elem" in give me memory error.

#include <iostream>  class string {     int sz;     char* elem;      public:         string()         : sz(0)         {  // default constructor          }          string(char a[]) {             int counter = 0;             while(a[counter]) {                 counter++;                 sz++;             }          }          int size() {             return sz;         }          void push_back(char a) {          }  };  int main() {     string s1("hello");     std::cout << s1.size() << "\n";     return 0; } 

this runs , works fine on compiler, , @ ideone.com, think it's purely accident. thing didn't initialise sz in constructor takes char[] array, , member variables aren't zero-initialised default. either initialise sz in second constructor way did in first, or do:

class string {     int sz = 0; // can c++11 onwards     char* elem;      public:     // on.