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Last updated：2026-10-13

C++ Memory Leak Detection

How to check memory, from C++那些事

When we write C/C++ code, memory problems are everywhere; both at work and in interviews, the topic of memory is inescapable.

Today, using one example, we will get hands-on with valgrind for checking C++ memory, step by step.

Three problems are simulated:

  • Detecting an invalid memory release
  • Detecting a write overflow
  • Detecting a read overflow

You will learn:

  • How to detect these three problems
  • How to debug with gdb + valgrind together

Feel free to join the discussion.

1. A sample to start with

Take a look at the code below; we will find quite a few problems:

  • strcpy is unsafe — no matter how large the memory is, it may overflow
  • msg is accessed out of bounds when printing
  • The destructor does not release memory

Doesn't it look just like code you once wrote yourself? Ha.

This section will detect these problems.

class Message {
private:
    char *msg;

public:
    Message(const char *text) {
        msg = new char[3];
        std::strcpy(msg, text);
    }

    void printOutOfRange() {
        std::cout << msg[5] << std::endl;
    }

    ~Message() {

    }
};

int main() {
    Message message("hello");
    message.printOutOfRange();

    return 0;
}

2.valgrind

Installing valgrind:

brew install valgrind // mac
sudo apt-get install install // ubuntu

For installation on more systems, see the official site:

https://valgrind.org/downloads/

3. Usage

First, when compiling, it is recommended to use debug, -g, so that you can detect which line of the code has the problem.

g++ val_test.cc -g

Then we execute:

valgrind --leak-check=full ./a.out

We then get the information valgrind gives us:

C++ Memory Leak Detection | C++那些事

At this point, we can see several problems above:

  • Invalid write

Occurs at val_test.cc::11

  • Invalid read

Occurs at val_test.cc:10

  • Memory leak

1 allocs, 0 frees, 3 bytes allocated.

This information lets us quickly locate which line of code has the problem.

4. Debugging with gdb

When we add --vgdb-error=0, we can debug the code very conveniently — debugging on one side while seeing where the problems are.

valgrind --leak-check=full --vgdb-error=0 ./a.out

C++ Memory Leak Detection | C++那些事

Afterwards, we attach with gdb

gdb -q a.out

Paste the target line above into gdb, then continue, and we can see the code for the corresponding line along with the further information valgrind keeps printing.

C++ Memory Leak Detection | C++那些事

C++ Memory Leak Detection | C++那些事

This section walked you through memory problems in practice. If it helped you, feel free to share it.

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