Valgrind不报告“删除数组”上的内存泄漏

Question

After implementing the C++ code below, I ran valgrind --leak-check=full in order to check if there was any memory leak. The result was 0 bytes were in use at exit and no leaks are possible .

However, later I discovered that I've forgotten to use delete[] x instead of just delete x inside the destructor.

I searched for some explanations (for instance: delete vs delete[] operators in C++ ), and everything I read said that using delete without [] can cause memory leak, since it calls just the destructor for the first object in the array.

I changed the code to delete[] and the valgrind output was the same (as expected). But now I'm confused: "Is there a problem with valgrind, or does delete really works fine for arrays even without the operator [] ?"

#include <iostream>
#include <string.h>
using namespace std;
class Foo {
  private: char *x;
  public:
    Foo(const char* px) {
       this->x = new char[strlen(px)+1];
       strcpy(this->x, px);
    }
    ~Foo() {
       delete x;
    }
    void printInfo() {  cout << "x: " << x << endl;  }
};
int main() {
   Foo *objfoo = new Foo("ABC123");
   objfoo->printInfo();
   delete objfoo;
   return 0;
}

Answer 1

using delete without [] causes memory leak.

No, it causes Undefined Behavior .

Your program where you allocate using new [] and deallocate using delete has Undefined Behavior. Actually, you are lucky(rather unlucky) it doesn't show some weird behavior.

As a side note, You also need to follow the Rule of Three for your class. Currently, you don't do so and are calling for trouble in near future.

Answer 2

The main difference between delete and delete[] is not about memory deallocation, but about destructor calls.

While formally undefined behavior , in practice it will more or less work... except that delete will only call the destructor of the first item.

As a side note, you may get an issue:

#include <iostream>

struct A{
  ~A() { std::cout << "destructor\n"; }
};

int main() {
  A* a = new A[10];
  delete a;
}

a bit like:

*** glibc detected *** ./prog: munmap_chunk(): invalid pointer: 0x093fe00c ***
======= Backtrace: =========
/lib/libc.so.6[0xb759dfd4]
/lib/libc.so.6[0xb759ef89]
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/libstdc++.so.6(_ZdlPv+0x21)[0xb77602d1]
./prog(__gxx_personality_v0+0x18f)[0x8048763]
./prog(__gxx_personality_v0+0x4d)[0x8048621]
======= Memory map: ========

See it at ideone .