Why doesn't overloading operator new with std::string work with msvc /MD flag

Question

I have some code that looks like this.

#include <iostream>
#include <string>
void* operator new(std::size_t n){
    std::cout << "[Allocating " << n << "bytes] ";
    return malloc(n);
}

void operator delete(void* pointer) throw() {
    free(pointer);
}

int main(){

    for(int i = 0; i < 24; i++){
        std::cout << i << ": " << std::string(i, '=') << std::endl;
    }

}

this compiles and runs using gcc compiler and also runs when using msvc compiler with the flag /MT or /MTd set. however if I set the flag to /MDd or /MD The program crashes. It seems to be stuck in a recursion inside the new operator. Why doesn't this happen when compiling and linking against LIBCMT.lib which as I can tell from Microsoft docs is the difference between /MT and /MD.

Why is your operator delete marked throw and your operator new not? — catnip
– catnip, Commented Sep 20, 2019 at 23:12

SergeyA · Accepted Answer · 2019-09-20 20:32:08Z

1

You are using std::couts operator << inside your new replacement function. This operator easily might call new itself to perform some allocation, in turn, calling your new replacement, which would call <<, and so on, and so forth.

You should be careful about what you do in those functions. If you want to see how they are called, you can use some preallocated structures and log with them (or simply increment a global counter).

The reason why it happens with one set of flags but not the another is likely due to different allocation strategies employed by << based on those flags. Those would be private to implementation and you shouldn't rely on them.

answered Sep 20, 2019 at 20:32

SergeyA

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1 Comment

masonCherry Over a year ago

Thanks for this answer.

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