I'm just decided to test malloc and new. Here is a code:

#include <iostream>
#include <string>

struct C
{
    int a = 7;
    std::string str = "super str";
};

int main()
{
    C* c = (C*)malloc(sizeof(C));
    std::cout << c->a << "\n";
    std::cout << c->str << "\n";
    free(c);
    std::cout << "\nNew:\n\n";
    c = new C();
    std::cout << c->a << "\n";
    std::cout << c->str << "\n";
}

Why an output of this program stops at std::cout << c->a << "\n";:

-842150451

C:\Code\Temp\ConsoleApplication12\x64\Debug\ConsoleApplication12.exe (process 22636) exited with code 0.

Why does compiler show no errors - I thought, std::string isn't initialized properly in case of malloc, so it should break something. If I comment out printing of the string, I'm getting a full output:

-842150451

New:

7
super str

C:\Code\Temp\ConsoleApplication12\x64\Debug\ConsoleApplication12.exe (process 21652) exited with code 0.

I use MSVS2022.

CodePudding user response：

You've used malloc. One of the reasons to not do this is that it hasn't actually initialized your object. It's just allocated memory for it. As a result, when accessing member fields, you get undefined behavior.

You have also forgotten to delete the C object you created with new. But you may wish to use a std::unique_ptr in this scenario, to avoid having to explicitly delete the object at all. The smart pointer will automatically free the memory when it goes out of scope at the end of main.

auto c = std::make_unique<C>();

std::cout << c->a << std::endl;
std::cout << c->str << std::endl;

CodePudding user response：

(C*) is an Explicit Conversion. It tells the compiler to shut up and do exactly what it was told to do, no matter how stupid that may be. You don't get an error message because the code explicitly told the compiler not to give you one.

To allow the programmer and compiler the maximum amount of leeway, C will not prevent you from doing some pretty insane things. It assumes you know what you are doing and are not trying to shoot yourself.