Expression must have class type when overloading array index operator

Question

The code is below

The error is "Expression must have class type" on the line: if (index >= mElements.size())

But not on the line return mElements[index]

I dont understand why, is this a glitch?

struct Matrix4x4
{
    float mElements[16];

    Matrix4x4()
    {
        memset(&this->mElements, 0, sizeof(this->mElements));
        this->mElements[0] = 1;
        this->mElements[5] = 1;
        this->mElements[10] = 1;
        this->mElements[15] = 1;
    }

    Matrix4x4(const float (&mElements)[16])
    {
        for (int i = 0; i < 16; i++)
            this->mElements[i] = mElements[i];
    }

    float& operator[](int index)
    {
        if (index >= mElements.size()) {
            printf("Array index out of bound, exiting");
            return 0.0F;
        }
        return mElements[index];
    }
}

You can't do `mElements.size()` in C++. plain arrays don't have any functions. — NathanOliver, Mar 18 '21 at 21:27
Alternative: use [`std::array mElements;`](https://en.cppreference.com/w/cpp/container/array) as your member, and adjust accordingly. — WhozCraig, Mar 18 '21 at 21:33
can't you do sizeof(array)/sizeof(array[0])? It will gets you the length try once — Syed Mohib Uddin, Mar 18 '21 at 21:37
@SyedMohibUddin I think that this is not the best way to get an array length, due that there's a standard array instantiation on the STL, but it solves the problem in many cases — , Mar 18 '21 at 21:49
Yeah there are many ways one is pointer hack *(&arr + 1) - arr — Syed Mohib Uddin, Mar 18 '21 at 21:50

score 2 · Answer 1 · answered Mar 18 '21 at 21:27

2

Plain arrays don't have .size(). Use std::size(mElements) from #include <iterator>, or std::extent_v<decltype(mElements)> from #include <type_traits>.

answered Mar 18 '21 at 21:27

HolyBlackCat

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score 0 · Answer 2 · answered Mar 18 '21 at 21:39

mElements is a plain old array. They don't have member functions. Hence, mElements.size() is not accepted by the compiler.

With the introduction of std::array in C++11, there is less of a need to use plain arrays.

Replace

float mElements[16];

with

std::array<float, 16> mElements;

Then, you can use mElements.size().

Expression must have class type when overloading array index operator

2 Answers2