typedef in C++ code confusion

Question

I came across some code with various typedefs as follows:

class GAGenome : public GAID {
public: GADefineIdentity("GAGenome", GAID::Genome);

public:
  typedef float (*Evaluator)(GAGenome &);
  typedef void  (*Initializer)(GAGenome &);
  typedef int   (*Mutator)(GAGenome &, float);
  typedef float (*Comparator)(const GAGenome&, const GAGenome&);
  typedef int   (*SexualCrossover)(const GAGenome&, const GAGenome&, 
               GAGenome*, GAGenome*);
  typedef int   (*AsexualCrossover)(const GAGenome&, GAGenome*);
//some other code

I don't understand the 'typedef' usage here, so can anyone teach me what does it mean? It looks a little bit complex here.

The first problem you have is that this code is written in C++, not C (as the question was originally tagged). The colons and the public keywords alone indicate that; the omnipresent & for references is also indicative. — Jonathan Leffler
– Jonathan Leffler, Commented Sep 12, 2013 at 2:01
I'm glad you're only asking about the typedefs; the bit that has me puzzled is the meaning of the GADefineIdentity("GAGenome", GAID::Genome); line. If it is a macro (perish the thought), then maybe it makes sense; but as code in a class definition ... I'm puzzled. — Jonathan Leffler
– Jonathan Leffler, Commented Sep 12, 2013 at 2:10

Jonathan Potter · Accepted Answer · 2013-09-12 02:07:34Z

2

Those lines are defining types that can be used as pointers to functions.

typedef float (*Evaluator)(GAGenome &);

This defines the Evaluator type as a pointer to a function that takes a reference to GAGenome as its single parameter, and returns a float.

You could use it like this:

float my_Evaluator_Function(GAGenome& g)
{
    // code
    return val;
}

GAGenome::Evaluator pfnEval = my_Evaluator_Function;
float val = pfnEval(myGenome);

answered Sep 12, 2013 at 2:07

Jonathan Potter

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Jonathan Leffler · Accepted Answer · 2013-09-12 02:06:11Z

1

All six typedef declarations specify pointers to functions of various sorts.

The first says that a variable of type GAGenome::Evaluator is a pointer to a function that takes a (non-constant) GAGenome reference and returns a float value. That is, given:

GAGenome x = ...suitable initializer...;
GAGenome::Evaluator e = ...suitable function name...;

float f = e(x);  // Call the function whose pointer is stored in e

The other function pointer types are similar, each with their own slightly different meaning due to the different return type or sets of parameters.

answered Sep 12, 2013 at 2:06

Jonathan Leffler

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