implicit initialization

fcarney

I am trying to determine if structs will be initialized if defined outside a function in a compilation unit:

...
typedef struct SomeStruct{
    int somevalue;
} buncha_structs[256];
...
int main(){
    SomeStruct local_structs[256];
}

If I am understanding this correctly. Then bunch_structs is initialized, but local_structs are not and require explicit initialization. Is this true? I am having trouble testing this out. I can test this out with the new operator by creating a memory pool and feeding a pointer to that pool to new(pool). Cannot think of a way to test this with a struct array outside of a function.

JonB

@fcarney
Yes, and also as per reading the link you referenced, your understanding is correct. Static/global storage declarations will be initialized to 0, but local/stack will be uninitialized. This would be just as true if your variable was int instead of struct ....

I'm not sure why you cannot think how to test this. I have gone to http://cpp.sh and pasted:

// Example program
#include <iostream>
#include <string>

struct SomeStruct
{
    int somevalue;
} buncha_structs[256];

int main()
{
    SomeStruct local_structs[256];
    
    std::cout << local_structs[1].somevalue << std::endl;

    std::cout << buncha_structs[1].somevalue << std::endl;
}

available as http://cpp.sh/5gbae (don't know how long that link is valid).

Note that I think you did not intend the typedef in your code. With the typedef, your buncha_structs is a type not a variable. I have removed the typedef to give you a global variable, which I think is what you intended.

The above reliably outputs 0 for the second, global variable case, but outputs a "random", changing number each time for the first, local case. You should also see that it shows an "uninitialized" warning message against your use in the local variable case.

fcarney

One issue I ran into was my local variables were showing zeros as well. It wasn't until I inited the ram with 255 for every character and used new(mempool()) to use that preinited ram that I did see uninitialized ram. Of course that was on the heap and not the stack. I did test this over the weekend and observed what you said. Variables declared outside of local scope are initialized to zero. While stack and heap are not.