将模板类型传递给函数并用于局部变量赋值 C++

Question

我有以下代码：

template<typename T> void computeFractalDimensionData(RandomWalkMethods::LatticeType latticeType, gsl_rng* randNumGen) {

    int nD = 0;

    // if T is of type std::pair<int,int> then set no. of dimensions to 2
    if (typeid(T) == typeid(std::pair<int, int>)) {
        nD = 2;
    }

    // else if T is of type RWM::Triple<int,int,int> then set no. of dimensions to 3
    else if (typeid(T) == typeid(RandomWalkMethods::Triple<int, int, int>)) {
        nD = 3;
    }

    else {
        return;
    }

    // Create vector of T structs to store DLA structure results
    std::vector<T> aggResults;

    // Initialise particle spawning type and attractor type for DLA system
    RandomWalkMethods::ParticleSpawnType spawn = RandomWalkMethods::CONSTANT_RANDOM_BOUNDINGBOX_EDGE;
    RandomWalkMethods::AttractorDLAType attractor = RandomWalkMethods::POINT;

    // Under-estimate for fractal dimension of the DLA
    const double fractalDimUnderestimateRecip = 1 / 1.65;

    for (int i = 100; i <= 1000; i += 100) {

        // initialise spawnDiameter using: exp(log(n)/fDUR) = n^{1/fDUR}
        int spawnDiam = 2*static_cast<int>(std::pow(i, fractalDimUnderestimateRecip));

        // if system is 2-dimensional, compute DLA for 2D on given lattice
        if (nD == 2) {
            aggResults = RandomWalkMethods::diffusionLimitedAggregateRandomWalk2D(i, spawn, spawnDiam, latticeType, randNumGen, attractor);
        }

        // else if system is 3 dimensional, compute DLA for 3D on given lattice
        else if (nD == 3) {
            aggResults = RandomWalkMethods::diffusionLimitedAggregateRandomWalk3D(i, spawn, spawnDiam, latticeType, randNumGen, attractor);
        }

        // compute the minimum bounding radius which encloses all particles in the DLA structure
        double boundingRadius = std::sqrt(maxMagnitudeVectorOfMultiples< double, T >(aggResults));

    }


}

我可以用

这样的语句调用

computeFractalDimensionData< std::pair<int,int> >(lattice, randNumGen);

或

computeFractalDimensionData< RandomWalkMethods::Triple<int,int,int> >(lattice, randNumGen);

其中 Triple 只是我用 3 个元素定义的结构（与 std::pair 基本相同，但扩展了 3 个字段）。另外，函数 diffusionLimitedAggregateRandomWalk2D 和 diffusionLimitedAggregateRandomWalk3D return 类型分别为 std::vector<std::pair<int,int>> 和 std::vector<Triple<int,int,int>>。

问题是，当我用上面的任一语句调用时，我得到以下错误（发生在赋值语句 aggResults = ...）：

binary '=': no operator found which takes a right-hand operand of type 'std::vector<std::pair<int,int>,std::allocator<_Ty>>' (or there is no acceptable conversion)

和 Triple<int,int,int> 的情况类似。据我了解，这意味着我需要为这 2 个结构重载赋值运算符 - 但是我不认为这是这里的问题，因为之前在我的程序中正确使用了以下语句：

std::vector< std::pair<int,int> > aggResults = RandomWalkMethods::diffusionLimitedAggregateRandomWalk2D(nParticles, boundingBox, spawnDiam, latticeType, randNumGen, attractor, &diffLimAggFile);

所以我知道我可以将 DLA 方法的结果分配给正确类型的变量，但是如果我通过使用将类型传递给模板函数来尝试它，编译器会抱怨，如上所示。

这里发生了什么，我将如何解决这个问题？

Answer 1

这是因为

aggResults = diffusionLimitedAggregateRandomWalk2D(i, spawn, spawnDiam, latticeType, randNumGen, attractor);

with aggResults being a std::vector<T> 即使 T is Triple<int, int, int> but diffusionLimitedAggregateRandomWalk2D returns a std::vector<std::pair<int, int>> 也会被编译.

建议的解决方案：声明一个模板化函数并将其专门用于某些 T.

template<typename T>
void computeFractalDimensionData(RandomWalkMethods::LatticeType latticeType, gsl_rng* randNumGen);

template<>
void computeFractalDimensionData<std::pair<int, int>>(RandomWalkMethods::LatticeType latticeType, gsl_rng* randNumGen)
{
    // ...
}

template<>
void computeFractalDimensionData<Triple<int, int, int>>(RandomWalkMethods::LatticeType latticeType, gsl_rng* randNumGen)
{
    // ...
}

它使代码更具可读性并且无法编译以下行并出现帮助编译错误：

computeFractalDimensionData<void>(lattice, randNumGen);

Answer 2

YSC的方案不错。我希望您注意函数中的以下代码是对模板的错误使用：

    // if system is 2-dimensional, compute DLA for 2D on given lattice
    if (nD == 2) {
        aggResults = RandomWalkMethods::diffusionLimitedAggregateRandomWalk2D(i, spawn, spawnDiam, latticeType, randNumGen, attractor);
    }

    // else if system is 3 dimensional, compute DLA for 3D on given lattice
    else if (nD == 3) {
        aggResults = RandomWalkMethods::diffusionLimitedAggregateRandomWalk3D(i, spawn, spawnDiam, latticeType, randNumGen, attractor);
    }

模板用于静态多态，而您在模板函数中使用动态代码（这些if (nd == ...)）。正确使用静态多态性可以引入模板参数 dimension.

将模板类型传递给函数并用于局部变量赋值 C++

Issue passing template type to function and using for local variable assignment c++

c++

templates

assignment-operator

template-function