c++ User defined conversion - implicit conversion [duplicate] - c++

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I have a question about user defined conversion.
class String {
char* m_data;
public:
String(): m_data(NULL) {}
String(const char* cstr): m_data(new char[strlen(cstr)+1]) {
strcpy(m_data, cstr);
}
~String() {
delete[] m_data;
}
String& operator=(const char* cstr) {
delete[] m_data;
m_data = new char[strlen(cstr)+1];
strcpy(m_data, cstr);
return *this;
}
operator const char*() const {
return m_data;
}
};
While this works:
int main(int argc, char** argv) {
String a;
String b;
a = "aaa";
b = (const char *)a;
return 0;
}
This does not:
int main(int argc, char** argv) {
String a;
String b;
a = "aaa";
b = a;
return 0;
}
I get double free or corruption runtime error. Valgrind says something about invalid delete.
Why do I have to explicitly typecast it? I thought it would work this way with explicit operator const char*(). Am I doing something wrong?

Answer 1

You forgot to define copy assignment operator:
String& operator=(const String& other) {
if(this != &other) {
char* new_data = new char[strlen(other.m_data)+1];
strcpy(new_data, other.m_data);
delete[] m_data;
m_data = new_data;
}
return *this;
}
Because of that your compiler had to define "default" copy assignment operator, which simple assigns all fields of "other" to a current object:
String& operator=(const String& other) {
m_data = other.m_data;
return *this;
}
So you have two pointers to same m_data in a and b and on the exit from main, delete[] will be called twice.