# C++ Reference: constraint_solver

This documentation is automatically generated.

Declaration of the core objects for the constraint solver.

The literature around constraint programming is extremely dense but one can find some basic introductions in the following links:
- http://en.wikipedia.org/wiki/Constraint_programming
- http://kti.mff.cuni.cz/~bartak/constraints/index.html

Here is a very simple Constraint Programming problem:

If we see 56 legs and 20 heads, how many two-legged pheasants and four-legged rabbits are we looking at?

Here is some simple Constraint Programming code to find out:
```   void pheasant() {
Solver s("pheasant");
// Create integer variables to represent the number of pheasants and
// rabbits, with a minimum of 0 and a maximum of 20.
IntVar* const p = s.MakeIntVar(0, 20, "pheasant"));
IntVar* const r = s.MakeIntVar(0, 20, "rabbit"));
// The number of heads is the sum of pheasants and rabbits.
IntExpr* const heads = s.MakeSum(p, r);
// The number of legs is the sum of pheasants * 2 and rabbits * 4.
IntExpr* const legs = s.MakeSum(s.MakeProd(p, 2), s.MakeProd(r, 4));
// Constraints: the number of legs is 56 and heads is 20.
Constraint* const ct_legs = s.MakeEquality(legs, 56);
DecisionBuilder* const db = s.MakePhase(p, r,
Solver::CHOOSE_FIRST_UNBOUND,
Solver::ASSIGN_MIN_VALUE);
s.NewSearch(db);
CHECK(s.NextSolution());
LOG(INFO) << "rabbits -> " << r->Value() << ", pheasants -> "
<< p->Value();
LOG(INFO) << s.DebugString();
s.EndSearch();
}
```
which outputs:
```   rabbits -> 8, pheasants -> 12
Solver(name = "pheasant",
state = OUTSIDE_SEARCH,
branches = 0,
fails = 0,
decisions = 0
propagation loops = 11,
demons Run = 25,
Run time = 0 ms)
```
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