在 C++ 中定义二维数组

Question

As per my understanding, we need to define a constant row and a constant column size for a 2d array in c++ unless we are dynamically allocating it.

I tried this piece of code and it worked without any errors.

    std::vector<std::string> strs;
    for(int i ; i < 10; ++i)
    {
       strs.push_back("This is a test"+ std::to_string(i)); 
    }
    int length = 0;
    for(int i ; i < 10; ++i)
    {
       if(strs[i].size() > length)
           length = strs[i].size();
    }

    char c[20][length + 1];
    for(int i ; i < 20; ++i)
    {
       memcpy(c[i], "Test", 5);
       cout << c[i] << endl;
    }

Could someone please explain how come defining a 2D character array with variable length is possible here?

Thanks K

Answer 1

char c[20][length + 1];

is not legal C++ if length is not a compile time constant. Some compilers, such as g++, allow it as an extension.