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Last updated：2026-10-12

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:

detect

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

attach

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.

gdb

continue

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

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