Can macros be used to access or assign values to elements in dynamic array or vector

Question

I have written a small code which isolates my flawed approach/understanding in using macros to access or assign values to elements in a std::vector. Below is the snippet of the code.

#define mat(i,j,nrows) mat[((j)*(nrows))+(i)]


struct _STR1
{
 int nRows, nCols;
 std::vector < double >mat;
 std::vector < double >anothermat;
};

void Create_Data (int &nC, _STR1 * &_str)
{
 _str = new _STR1[nC];

 for (int myid = 0; myid < nC; myid++)
 {
  _str[myid].nRows = 100;
  _str[myid].nCols = 3;

  _str[myid].mat.resize (_str[myid].nRows * _str[myid].nCols);

  _str[myid].anothermat.resize (_str[myid].nRows * _str[myid].nCols);

  for (int i_row = 0; i_row < _str[myid].nRows; i_row++)
    {
      _str[myid].mat (i_row, 0, _str[myid].nRows) = 1.0e0;

      _str[myid].mat (i_row, 1, _str[myid].nRows) = 1.0e0;

      _str[myid].mat (i_row, 2, _str[myid].nRows) = 1.0e0;

      _str[myid].anothermat (i_row, 2, _str[myid].nRows) = 1.0e0;
    }
 }
}

If I comment "_str[myid].anothermat (i_row, 2, _str[myid].nRows) = 1.0e0;", I do not get any error. Otherwise I get the following error

error: call of an object of a class type without appropriate operator() or conversion functions to pointer-to-function type

I think my understanding in using a macro is wrong but I am unable to understand why this is so.

Can anyone please tell me why this approach is wrong and why I have the error in one case whereas the other does not. Is my usage of macro correct?

Unrelated to your question and problem, but names beginning with leading underscore and followed by an upper-case letter (like for example `_STR1`) are *reserved* for the "implementation" (compiler and the standard library). Don't use such names in your own code. See e.g. [this QA for more details](https://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier). — Some programmer dude, Sep 12 '18 at 06:32
You're not using a macro. Are you sure that's the only line you get an error on, what about the line above it? — Tas, Sep 12 '18 at 06:32
As for your error, `_str[myid].anothermat` is a *vector*. It doesn't have a function-call operator. What are you really trying to do there? What is the purpose of that line? Same with the lines using the vector `mat`? — Some programmer dude, Sep 12 '18 at 06:33
Ooops I am sorry @Someprogrammerdude. I have added the macro now — Suman Vajjala, Sep 12 '18 at 06:35
Why are you naming your macro with the same name as the vector you are trying to access? You should not be doing that. — Remy Lebeau, Sep 12 '18 at 06:35
With that macro and the code as you show it now, you should get a *lot* of more errors, since *all* uses of the symbol `mat` would be the macro. Even the declaration of the *member variable* `mat` inside your structure. There's a reason macros are usually all upper-case while variables and functions are not. — Some programmer dude, Sep 12 '18 at 06:36
And to solve your problem, ***don't use macros!*** Create a simple (possibly templated) function which does what you want. — Some programmer dude, Sep 12 '18 at 06:37
@Someprogrammerdude Yes. But with the following code, I do not get error if I comment out a single statement and the code runs. I understand that my logic is wrong? But why doesnt the compiler issue an error? — Suman Vajjala, Sep 12 '18 at 06:38
If you only defined mat as a macro, why would you expect it to work for anothermat? That's why you are only getting an error on that line. In any case, you should give a different name to your macro - not the same as the member in the struct. And better yet, use functions instead of macros. — Gonen I, Sep 12 '18 at 06:38
Will the code be correct if I comment this statement out- "_str[myid].anothermat (i_row, 2, _str[myid].nRows) = 1.0e0;" — Suman Vajjala, Sep 12 '18 at 06:42
@Someprogrammerdude Thank you for your comments and advice. I am not an expert in programming, so I make these mistakes. I wanted to ask you if I comment this statement or line "_str[myid].anothermat (i_row, 2, _str[myid].nRows) = 1.0e0;" will my code be correct? Will it work the way it is supposed to work? — Suman Vajjala, Sep 12 '18 at 06:49
@RemyLebeau What will happen if I name the macro with the same name as the vector? Is this bug? — Suman Vajjala, Sep 12 '18 at 06:54
Your struct name _STR1 is undefined behaviour (https://en.cppreference.com/w/cpp/language/identifiers) as already mentioned by @someprogrammerdude. Personally I strongly dislike the _python++ style of names. — Paul Floyd, Sep 12 '18 at 12:11

score 1 · Answer 1 · edited Sep 12 '18 at 12:03

1

Macros are considered bad practice.

Use a function instead. They are much less error prone (e.g. type safe).

You can use various techniques to have the same performance as with C macros, such as template functions.

In your case, I would simply define a wrapper class:

template <typename T>
class Mat {
  private:
    std::vecotr<T> mat;
    int nrows;
  public:
    Mat(const Matrix & mat, int nrows);
    const T& operator()(int i, int j) const {
        return mat[j * nrows + i];
    }
    T& operator()(int i, int j) {
        return mat[j * nrows + i];
    }
};

edited Sep 12 '18 at 12:03

Jarod42

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answered Sep 12 '18 at 06:42

schorsch312

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1

Your pseudo code is a bit too pseudo. Though the idea is sound. Also `getMat` can be `operator()`, so the OP won't need to change their code too much. – StoryTeller - Unslander Monica Sep 12 '18 at 06:44
Thank you. After I discovered the bug, this thought came to my mind. I am not a programming expert so I make these mistakes! Thank you very much – Suman Vajjala Sep 12 '18 at 06:45
I corrected the template placement and added the operator overload. – schorsch312 Sep 12 '18 at 06:53
Sorry to be a pain, but you also have incorrect matrix position calculation. And convention has matrices record the number of columns, not the number of rows. – Gonen I Sep 12 '18 at 07:58

score 0 · Answer 2 · answered Sep 12 '18 at 12:12

Though your approach is unconventional, it's not technically wrong in the case of the first matrix. In the case of the second matrix you simply didn't write a macro for it, so anothermat (i_row, 2, _str[myid].nRows) doesn't get converted to anything, and since it's not valid C++ on its own your program can't be built.

schorsch312's answer shows a better approach (despite the typos); as it happens, I wrote something similar just yesterday.

Can macros be used to access or assign values to elements in dynamic array or vector

2 Answers2