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Thread: Different sizeof array when I insert it by myself

  1. #1
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    Apr 2019
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    Different sizeof array when I insert it by myself

    I'm just struggling with winding numbers algorithm. I have a homework to write a program where I have to insert points by myself of polygon and points which I'm going to investigate.
    Anyway, the problem is I get a different output of sizeof(polygon) when I insert it by myself (4) than it is written in code (48). I dunno why and I'm struggling with this problem. I would be very grateful if someone could help!

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  2. #2
    2kaud's Avatar
    2kaud is offline Super Moderator Power Poster
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    Re: Different sizeof array when I insert it by myself

    Please post formatted code using code tags rather than a screen image.

    I think you're asking why the values of n1 and n2 in the code below are not the same?

    Code:
    struct Point {
    	int x;
    	int y;
    };
    
    
    int main()
    {
    	Point polygon1[] = { {2,2}, {4,4}, {6,6}, {-3,1}, {-1,-1}, {5,1} };
    
    	const size_t n1 = sizeof(polygon1) / sizeof(polygon1[0]);
    
    	cout << "n1 = " << n1 << endl;
    
    	Point* polygon2 = new Point[6];
    
    	for (size_t i = 0; i < 6; ++i)
    		cin >> polygon2[i].x >> polygon2[i].y;
    
    	const size_t n2 = sizeof(polygon2) / sizeof(polygon2[0]);
    
    	cout << "n2 = " << n2 << endl;
    }
    giving
    Code:
    n1 = 6
    1 1 2 2 3 3 4 4 5 5 6 6
    n2 = 0
    n1 is the expected value of 6.

    However n2 is 0. The reason for this is that sizeof(polygon2) gives the size of the pointer - not the size of the memory to which polygon2 points. For a 32-bit compile, the size of a pointer is 32 bits (4 bytes) - so you get 4 / 8 which is 0.

    Rather than using dynamically allocated memory, it would be better to use a vector if you don't know at compile time the size of the array.

    Consider:

    Code:
    struct Point {
    	int x;
    	int y;
    
    	Point(int x1, int y1) : x(x1), y(y1) {}
    };
    
    ...
    
    	vector<Point> polygon3;
    
    	for (size_t i = 0; i < 6; ++i) {
    		int x, y;
    		cin >> x >> y;
    		polygon3.emplace_back(x, y);
    	}
    
    	cout << "n3 = " << polygon3.size() << endl;
    This gives the output:

    Code:
    1 1 2 2 3 3 4 4 5 5 6 6
    n3 = 6
    Last edited by 2kaud; April 22nd, 2019 at 05:52 AM.
    All advice is offered in good faith only. All my code is tested (unless stated explicitly otherwise) with the latest version of Microsoft Visual Studio (using the supported features of the latest standard) and is offered as examples only - not as production quality. I cannot offer advice regarding any other c/c++ compiler/IDE or incompatibilities with VS. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/ and can be used without reference or acknowledgement. Also note that I only provide advice and guidance via the forums - and not via private messages!

    C++17 Compiler: Microsoft VS2019 (16.4.0)

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