data/src/OpenFOAM/fields/expression/expressionEval.H

/*---------------------------------------------------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Copyright (C) 2026 OpenFOAM Foundation
     \\/     M anipulation  |
-------------------------------------------------------------------------------
License
    This file is part of OpenFOAM.

    OpenFOAM is free software: you can redistribute it and/or modify it
    under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
    for more details.

    You should have received a copy of the GNU General Public License
    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.

Namespace
    Foam

Description
    A function which evaluates an expression into a temporary field. Note that
    this is in the top-level namespace, not the expression sub-namespace. This
    is because this is designed to be used in high-level code, so it is
    desirable to avoid the clutter associated with the sub-namespace.

\*---------------------------------------------------------------------------*/

#ifndef expressionEval_H
#define expressionEval_H

#include "expressionContainers.H"
#include "expressionResult.H"

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

namespace Foam
{

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

//- Base class for conditional expression evaluation
template<class Expression, class = void>
struct Eval;

//- If the argument is not an expression return it
template<class Expression>
struct Eval<Expression, EnableIfNotExpression<Expression>>
{
    static const Expression& eval(const Expression& e)
    {
        return e;
    }
};

//- If the argument is an expression evaluate it and return the result
template<class Expression>
struct Eval<Expression, EnableIfExpression<Expression>>
{
    static tmp<ResultType<Expression>> eval(const Expression& e)
    {
        return tmp<ResultType<Expression>>(new ResultType<Expression>(e));
    }
};

//- If the argument is an expression evaluate it and return the tmp<result>
//  otherwise return it
template<class Expression, class = EnableIfExpressionable<Expression>>
tmp<ResultType<Expression>> eval(const Expression& e)
{
    return Eval<Expression>::eval(e);
}

//- Evaluate an expression and write to Ostream
template<class Expression, class = EnableIfExpression<Expression>>
Ostream& operator<<(Ostream& os, const Expression& e)
{
    os << eval(e);
    return os;
}


// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

} // End namespace Foam

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

#endif

// ************************************************************************* //