C++派生类构造函数

Question

I'm learning C++ inheritance and i have some problems with it. I'm using Zinjal IDE with GNU c++ compiler.

This is the code:

class String {
protected:
    char str[MAX_STR_SIZE];
public:
    String(char str_init[])
    {
        strcpy(str, str_init);
    }
};

class PString : String
{ 
    public:
    PString(char str_init[]) 
    {
        if (strlen(str_init) > MAX_STR_SIZE)
            strncpy(str, str_init, MAX_STR_SIZE-1);
        else
            String(str_init); 
    }
};

Well it's just creating my own "string" class. But: What is the problem? The string can go too much bigger. So when the constructor of my "String" Class is called with a string the superssed "MAX_STR_SIZE" (which is defined as 80 chars for expamle) the program will crash with an array overflow ( >80 chars). So i want to create a "child" or "derived" class called "PString" which can handles overflow. As you can see PString child Class constructor checks the string if it's > MAX_STR_SIZE. If it's bigger than what char array can handle it cut off the string by MAX_STR_SIZE, so avoiding the overflow. If its smaller than MAX_STR_SIZE it calls the Parent class constructor. But, g++ fails telling me "No matching function call to 'String::String()'.

It's a lame error i know, but i'm just learning

Thanks in advance.

Answer 1

You can't just call a constructor out of the blue. There are only specific places you can call a constructor and this isn't one of them.

The compiler sees String(str_init); and ssumes it is a function call but you don't have a matching function - hence the error.

As per the comment below there is a subtlety here. The error message is "No matching function call to 'String::String()". In a class called String a method called String is going to be a constructor. Therefore the line String(str_init); isn't a function call, it is trying to make a String item using a non-existent default constructor.

Answer 2

what you are trying to do does not require this set of complications. In general strncpy will handle both cases correctly. However, for the education sake here is some analysis

1.In your constructor of the PString you call the construct of the String and ask it to strcpy str_intit into str. So, you cause the same memory corruption you wanted to avoid. you are missing String() constructor. I suggest to create the default constructor for the String and remove the one you use from PString.

 String() {str[0] = 0;}
 ..
 PString(char str_init[])
 {

2.your case could be modified a bit as the following:

if (strlen(str_init) > MAX_STR_SIZE) {
  strncpy(str, str_init, MAX_STR_SIZE-1);
  str[MAXLEN-1] = 0; // strncpy will not add '0' in this case. you need to do it yourself
}
else
  strcpy(str, str_init);

above, in the if clause you use str n cpy whether after the 'else' you can use the simple strcpy. So, it looks like this:

#include <cstring>
#define MAX_STR_SIZE 80

class String {
protected:
  char str[MAX_STR_SIZE];
public:
  String(char *str_init)
  {
    strcpy(str, str_init);
  }
  String() {
   str[0] = 0;
  }
};

class PString : String
{ 
public:
  PString(char str_init[])
  {
    if (strlen(str_init) > MAX_STR_SIZE) {
      strncpy(str, str_init, MAX_STR_SIZE-1);
      str[MAX_STR_SIZE-1] = 0;
    }
    else
      strcpy(str, str_init);
  }
};

Answer 3

2 Errors. First, you need to invoke the base constructor via your class' initializer list as explained in this answer .

Second you need to assign some object to your class when calling

String(str_init); 

The following code compiles on my machine.

#include <cstring>
#define MAX_STR_SIZE 20
class String {
protected:
    char str[MAX_STR_SIZE];
public:
    String(char str_init[])
    {
        std::strcpy(str, str_init);
    }
};

class PString : String
{ 
    public:
    PString(char str_init[]) : String(str_init) 
    {
        if (strlen(str_init) > MAX_STR_SIZE)
            strncpy(str, str_init, MAX_STR_SIZE-1);
        else
            String s = String(str_init); 
            String s2(str_init);
    int i = 0;
    i++;
    }
};