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Does template argument use runtime memory?

afedorov (59)
do template arguments use app memory?

For example:
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template<int Size>
class ArrayWithTemplate {
    int array[Size];
};

class ArrayWithConstructor {
    int *array;
public:
    ArrayWithConstructor(int size) : array{new int[size]} {}
};


int main() {
    ArrayWithTemplate<1> array1{};
    ArrayWithTemplate<10> array2{};

    ArrayWithConstructor arrayWithConstructor1{1};
    ArrayWithConstructor arrayWithConstructor2{10};
    return 0;
}



When I construct array1 variable, will the size of the array '1' use some memory during runtime?
Last edited on
Ganado (6784)
It's sort of an apples-to-oranges comparison because ArrayWithTemplate is allocated on the stack, and ArrayWithConstructor is [dynamically] allocated on the heap.

If I may, a better comparison would be the following:
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template<int Size>
class ArrayWithTemplate {
    int array[Size];
};

class ArrayWithHardcodedValue {
    int array[10];
};


ArrayWithHardcodedValue will look exactly the same as ArrayWithTemplate<10>, with no extra overhead, if that answers your question.

the ArrayWithTemplate<1> will not consume more memory than ArrayWithHardcodedValue with array[1] instead of [10].

It will most likely put a hardcoded value in the exe to say how much stack space to allow.

When I construct array1 variable, will the size of the array '1' use some memory during runtime?

You will inevitably have to put some value in a registry somewhere to make things function. Does that count as memory for you? It certainly doesn't need to load additional memory; putting a hardcoded value in a registry is about as basic of an operation as you can get.

ex, ArrayWithTemplate<100>:
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	subq	$432, %rsp
	.seh_stackalloc	432
	leaq	128(%rsp), %rbp
	.seh_setframe	%rbp, 128
	.seh_endprologue
	call	__main
	movl	$0, %eax
	addq	$432, %rsp

The 432 number correlates with the size of 100.

ArrayWithTemplate<1000> produces:
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	movl	$4032, %eax
	call	___chkstk_ms
	subq	%rax, %rsp
	.seh_stackalloc	4032
	leaq	128(%rsp), %rbp
	.seh_setframe	%rbp, 128
	.seh_endprologue
	call	__main
	movl	$0, %eax
	addq	$4032, %rsp

It has an extra check when it calls ___chkstk_ms, which I assume is some Microsoft safety check, but that's unrelated and trivial.
Last edited on
afedorov (59)
Thank you for response. It makes sense now.
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