warning "operation on <variable> may be undefined"

Question

When I use overloading [][] operators in c++ to create a minimal matrix class

class matrix {
private:
  vector<T> elems_;
  size_t         nrows_;
  size_t         ncols_;  
public:
  T const* operator[] ( size_t const r ) const { return &elems_[r * ncols_]; }
  T*       operator[] ( size_t const r )       { return &elems_[r * ncols_]; }  
  matrix ();
  matrix ( size_t const nr, size_t const nc ) 
    : elems_( nr * nc ), nrows_( nr ), ncols_( nc ) 
  { }
  matrix ( size_t const nr, size_t const nc, T const *data) 
    : elems_( nr * nc ), nrows_( nr ), ncols_( nc ) 
  { size_t ptr=0; 
    for (int i=0;i<nr;i++) 
        for (int j=0;j<nc;j++) 
             elems_[ptr] = data[ptr++]; 
  }
}

g++ returns the warning operation on ‘ptr’ may be undefined [-Wsequence-point]. In previous post Why I got "operation may be undefined" in Statement Expression in C++? it is explained that the last thing in the compound statement should be an expression followed by a semicolon but not which purpose it serves. Does it mean that g++ will always throw this warning for compound statements that have a for-loop at the end?

If so can anyone explain why this is useful? I cannot think of any reason why anyone should be advised against ending a compound statement with a for loop.

Answer 1

In elems_[ptr] = data[ptr++], it is undefined whether elems_[ptr] is evaluated first or data[ptr++] is evaluated first. This is so, because = does not introduce a sequence point.

Depending on the order, elems_[ptr] = data[ptr++] yields different results. Hence the warning.