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I am puzzled with the following piece of code where I am trying to understand the difference between getter/setter of the C++ bracket operator.

#include <iostream>
using namespace std;

template <class T, std::size_t N>
class Buffer
{
public:
    Buffer() : _pos(0) {}
    ~Buffer() {}
    //T& operator[](std::size_t pos) { cout << "\nSet\n"; return _data[pos]; }
    const T& operator[](std::size_t pos) const { cout << "\nGet\n"; return _data[pos]; }    
private:
    T _data[N];
    std::size_t _pos;
};

int main()
{
    Buffer<int, 10> b;
    int i = b[0];
    (void)i;
    return 0;
}

I am compiling it using GCC 4.9.3 (MinGW). When I run it, it prints "Get" as expected, since I am not changing the Buffer object in the assignment 

int i = b[0];
But if I un-comment the setter operator 
T& operator[](std::size_t pos)
then this time the program prints "Set". I cannot explain this behavior.

I have read many examples, including this one, where it is recommended to use a 'proxy' class, but then I do not understand why this approach is not used in the implementation of std::array like in the GCC implementation. My Buffer class is intended to be similar to std::array.

What am I doing wrong?

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yancheelo
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  • You're not doing anything wrong, but read the answers to the duplicate carefully. – juanchopanza Dec 21 '16 at 22:23
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    Hint: change `Buffer b;` to `const Buffer b;`. See any difference in your output? – simon Dec 21 '16 at 22:24
  • `T& operator[](std::size_t pos)` isn't different from `const T& operator[](std::size_t pos)` for this case, but you escape compile error, because second one actually got const argument `this`, by declaring it `const T& operator[](std::size_t pos) const`. Latter one is valid to use when the object it used on is const as well, in other cases compiler will prefer former. – Swift - Friday Pie Dec 21 '16 at 22:35
  • T& operator[](std::size_t pos) **it is** different from 
    const T& operator[](std::size_t pos) const
    . The former tells me that I can change the referenced value, the latter tells me that I can't do it. Please note the **const** attribute for the getter method.     – yancheelo Dec 21 '16 at 22:56

0 Answers0