Behaviour of #define macro expansion in C++

Question

I knew that if we don't put space after closing angle brackets in a variable declaration, C++ throws the following error.

‘>>’ should be ‘> >’ within a nested template argument list

But the error doesn't come if I use #define like in this code. Can someone explain me this?

I think #define is just a macro expansion and works like find-replace, so both the ways of declaring variable here should be identical.

Also this error doesn't occur if I compile it with C++11.

#include <bits/stdc++.h> using namespace std;  #define vi vector<int>  int main(){     //Doesn't work, compile error     vector<vector<int>> v;      //Works     vector<vi> vv; } 

Answer 1

Macro expansion happens after tokenisation; it doesn't replace text, but sequences of tokens.

This means that, with the macro, the expansion of vi gives a > token, separate from the one following the macro invocation. In each case, tokenisation only finds a single > character, so that's the resulting token.

Without a macro, the "greedy" tokenisation rule meant that the two consecutive characters were treated as a single >> token, until C++11 added a special rule for this case.