删除char指针在析构函数中不起作用

Question

I have a program where i ask the user for the name of a text file, I open the text file do stuff with it (read, write), then I close the file and exit the program.

Program.h

class Program
{
     char* fileName;
public:
     Program();
     ~Program();
     void ReadFile(void);
};

Program.cpp

Program::Program(){
     //contstructor
     fileName=NULL;
}

Program::~Program(){
     cout << "in destructor" ;
     delete []fileName;
}

void Program::ReadFile(void){
     fileName = new char[40];

     cout <<"Please enter the name of the file to open: ";
     cin.clear();
     cin.getline(fileName, 40);

     ifstream file (fileName);

     if(file.is_open()){
          //do stuff
     }
     file.close();
}

right now when I put delete []fileName; in the destructor it outputs ""in destructor" on the screen but fileName does not get deleted. If I take delete []fileName; and put it in ReadFile() after file.close() fileName gets deleted. Why is that?

The rest of my program works perfectly which is why none of that code is pasted. I am just trying to rid any memory leaks and fileName is the only one I am having trouble with so therefore I only pasted the code where fileName is used.

Any help is appreciated.

Additional information: I am using Visual Studio to write this and am using the Memory Leak Detection. This is what it outputs:

Detected memory leaks!
Dumping objects ->
{132} normal block at 0x005D49A0, 40 bytes long.
Data: 6E 61 6D 65 73 2E 74 78 74 00 CD CD CD CD CD CD
Object dump complete.
The program '[10772] program1.exe: Native' has exited with code 0 (0x0).

which is why I suspect delete []fileName; didn't work.

also, this is what the int main() looks like

int main(){
     Program abc;
     abc.ReadFile();
}

Oh, and Program.h can not be changed. Only .cpp can be change it's part of my requirements.

Answer 1

If filename is used only in readFile - then I advise you to remove it from Program class and make it automatic variable in that function:

void Program::ReadFile(void){
     char fileName[40];
     ...
     file.close();
     // no delete [] necessary
}

Your problem might be related to

1. Not initializing filename to nullptr in constructor
2. Not defining copy c-tor, assignment operator
3. Your are not deleting old filename in readFile

So, do not use your member variables as automatic variables to your methods.

If you have to have this member varible - change it to array - do not allocate it:

class Program {
private:
  // char* filename;
  char filename[40];
}; 

[UPDATE]

Your .h file is incorrect - it breaks rule of three (see http://en.wikipedia.org/wiki/Rule_of_three_(C%2B%2B_programming )) - copy constructor and assignment operator is missing. So be aware not to copy your Program class in any way of you cannot change this header.

After update only one thing is missing in your program:

Either add delete[] filename at the beginning of your readFile:

void Program::ReadFile(void){
     delete [] filename;
     fileName = new char[40];

Or (better) - do not re-allocate every time readFile is called:

void Program::ReadFile(void){
     if (!filename)
         fileName = new char[40];

Or (best) - allocate this memory in constructor only:

Program::Program() : filename(new char[40]) {}
void Program::ReadFile(void){
   // fileName = new char[40];

Answer 2

Are you sure your main is exactly as posted here? If you just define abc globally, it will be freed after memory dump report memory leaks and you may see invalid reports! You can insert a break point in the destructor and see if memory leaks reported after or before destructor

Answer 3

fileName clearly is getting deleted: the code in the destructor says so. But if the code you didn't show calls ReadFile more than once, the class will leak memory because each call to ReadFile allocates a new memory block and overwrites the pointer to the previous block.

Think RAII: Resource Allocation Is Initialization. In the constructor, allocate the memory block. In the destructor, delete it. Then ReadFile doesn't have to worry about allocating the block.

Or, even better, do as @PiotrNycz says, and change the pointer into an array. No need for dynamic allocation.