nested struct with array

Question

Please, help me to create a nested struct with an array. How do I fix this code?

class CMain
{
    public:
        CMain();
        ~CMain();

    private:
        struct
        {
            CCheckSum() : BufferSize(500) {memset(Buffer, 0, BufferSize);}
            const int BufferSize;
            char Buffer[BufferSize];
        }SmallBuffer;
}

Thanks.

Answer 1

Static arrays need to know their length at compile time, or you need to dynamically allocate memory:

struct CCheckSum
{
    CCheckSum()
    : BufferSize(500),
      Buffer(new char[BufferSize])
    {
        memset(Buffer, 0, BufferSize);
    }
    ~CCheckSum() { delete[] Buffer; } // Note the use of delete[]!
    const int BufferSize;
    char* Buffer;
}SmallBuffer;

You're probably better off using std::vector though:

struct CCheckSum
{
    CCheckSum() : Buffer(500, 0) {}
    std::vector<char> Buffer;  // A std::vector keeps
                               // track of its size enternally
}SmallBuffer;

Answer 2

Even though you declared BufferSize as const, at class scope this does not declare a compile-time constant, which is needed for declaring array sizes in C++, but a constant data member, which, once set in the constructor, cannot be changed anymore for the lifetime of the instance (but can still be different for each instance).

Two solutions: Make

1. BufferSize static const at class scope, or
2. (just) const at namespace scope (including global scope).

Answer 3

There is no problem related to the nesting.

Arrays have constant size. Either make BufferSize constant or use a std::vector instead of a char[].

Answer 4

Array size cannot be changed at run time. It should be known at compile time itself. You can use std::vector for this use case. Also, the struct name should be given before writing the constructor with the name CCheckSum.

struct CCheckSum
        {
            CCheckSum() : ....