memory allocation on the stack

This is a discussion on memory allocation on the stack within the C++ Programming forums, part of the General Programming Boards category; In C++, when a function is loaded are ALL of its statically allocated variables placed on the stack immediately, or ...

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    memory allocation on the stack

    In C++, when a function is loaded are ALL of its statically allocated variables placed on the stack immediately, or are local variables put on the stack only when they are in scope?

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    Algorithm Dissector iMalc's Avatar
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    The compiler calculates the size of the function's variables and upon entry the function adjusts the stack pointer by that much. At this point nothing is initialised though, it only contains garbage data.

    Then upon entry to each scope block (curly braces), variables inside that scope block are initialised (their constructor is called if one was provided).

    At the end of the scope block, those variables are destructed (their destructor is called if one was provided), but the stack space for them tends to exist until the end of the function.

    It's also possible for variables of non-overlapping scope to re-use the same portion of the stack.
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    Quote Originally Posted by iMalc View Post
    The compiler calculates the size of the function's variables and upon entry the function adjusts the stack pointer by that much....
    It's also possible for variables of non-overlapping scope to re-use the same portion of the stack.
    So in light of the last sentence, the size of the stack allocated for a function may be less than the sum of the sizes of its variables, correct?

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    Captain Crash brewbuck's Avatar
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    Variable lifetime matches variable scope, except in the case of static local variables, which have global lifetimes but local scope

    Quote Originally Posted by R.Stiltskin View Post
    So in light of the last sentence, the size of the stack allocated for a function may be less than the sum of the sizes of its variables, correct?
    Yes -- variables which have non-overlapping lifetimes might be placed at the same location in the activation record
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    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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    I don't think the standard actually says anything about how/when the space is allocated on the stack for local variables [or even that they have to be on the stack, as such - but some sort of stack-like construction is obviously necessary].

    In practice, what iMalc says holds true, but there is nothing saying the compiler is not ALLOWED to allocate more space on the stack in the middle of a function. It's just that the compiler would have to issue extra instructions to achieve this, and it's just making things more complicated than calculating the overall space needed.

    Of course, variables do not HAVE to reside on the stack at all. Local variables may well be stored entirely in registers.

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    Quote Originally Posted by matsp View Post
    Of course, variables do not HAVE to reside on the stack at all. Local variables may well be stored entirely in registers.

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    Heh. I hadn't thought of that at all when I posted the question but now that you mention it that seems pretty obvious.

    Thanks everyone.

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    Cat without Hat CornedBee's Avatar
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    I don't think the standard actually says anything about how/when the space is allocated on the stack for local variables [or even that they have to be on the stack, as such - but some sort of stack-like construction is obviously necessary].
    Or even if there is a stack. Yes, something stack-like is necessary, but a limited implementation could actually get by with something different.
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    Algorithm Dissector iMalc's Avatar
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    Yup 'sall true!
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