Question

I have been fiddling around with lambda expressions to learn how they work but came across an issue. I have been trying to figure what I am doing wrong but can’t seem to. The compiler just refuses to accept this simple example:

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  int a = 2;
  std::vector<int> vv(10);
  vv[2]=2;
  std::count( vv.begin(), vv.end(), [&a](int z) { return a == z; } );

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I get the error

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Error   1   error C2678: binary '==' : no operator found 
  which takes a left-hand operand of type 'int' (or there is no 
  acceptable conversion) e:\program files (x86)\microsoft visual studio
  11.0\vc\include\xutility 3243

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What am I doing wrong?

Answer 1

The algorithms need to know whether a predicate is based on comparing a value using equality or a unary predicate. To distinguish the two, a_if suffix is used for various algorithms: find_if(), `copy_if(), count_if(), etc. The lambda is OK but it isn’t equality-comparable to the value_type of the sequence. You need to use std::count_if() when using a predicate:

std::count_if( vv.begin(), vv.end(), [&a](int z) { return a == z; } );

… or a value:

std::count( vv.begin(), vv.end(), a);