C ++模板代码的语法和语义是什么？ [英] What are the syntax and semantics of C++ templated code?
问题描述
template<typename T, size_t M, size_t K, size_t N, typename std::enable_if_t<std::is_floating_point<T>::value, T> = 0>
void fastor2d(){//...}
我复制了这一行代码从cpp-reference（仅 std :: enable_if 部分，我确实需要 T 和所有三个 size_t ），因为我只想在使用float_types时才使用此函数...因此无法编译。
I copied this line of code from cpp-reference(only the std::enable_if part, i do need T and all three of the size_t's), because i would like to use this function only when floating_types are used on it ... it does not compile.
有人可以向我解释为什么，它甚至做什么？当我在使用它时，您以后如何调用此函数？
Could somebody explain to me, why, and what it even does? While i am at it, how do you call this function afterwards?
这里的每个教程或问题都被答案轰炸了，这很棒，但是对于那些不了解正在发生的事情，即使是那些并没有真正的帮助。（对不起，如果可能有点激动或激进）
Every tutorial or question here on SO gets bombed with answers, and that is great, but to someone who does not understand jacks*** of what is happening, even those are not really helpful.(sry, if possibly slightly agitated or aggressive)
编辑：我非常感谢所有人到目前为止的答案，我意识到我的措辞可能有点不对...我了解模板参数是什么，并且知道运行时和编译时等之间的区别，但是我只是无法很好地理解 std :: enable_if
i greatly appreciate all answers as of now, i realize that my wording might have been a bit off ... i understand what a template parameter is, and know the difference between runtime and compiletime etc, but i just cant get a good grasp of the syntax behind std::enable_if
EDIT2：
template<typename T, size_t M, size_t K, size_t N, typename = std::enable_if_t<std::is_integral<T>::value>>
void fastor2d(){
Fastor::Tensor<T,M,K> A; A.randInt();
}
这实际上是我唯一需要更改的东西。请注意random（）部分
This is literally the only thing i need changed. Notice the random() part
template<typename T, size_t M, size_t K, size_t N, typename = std::enable_if_t<std::is_floating_point<T>::value>>
void fastor2d(){
Fastor::Tensor<T,M,K> A; A.random();
}
推荐答案
我将尝试解释
模板参数是编译时参数（它们在运行时不会更改，您的申请时间）。函数参数是运行时的，并且具有内存地址。
Template arguments are compile time arguments (they do not change during the run-time of your application). Function arguments are run-time and have a memory address.
调用此函数将如下所示：
Calling this function would look something like this:
fastor2d<Object, 1, 2, 3>();
在<>括号中，您会看到编译时参数或更准确地说是模板参数，并且在这种情况下，该函数在（）中使用0个运行时参数。最后一个编译时参数具有默认参数，该默认参数用于检查函数是否应该完全编译（enable_if类型）。如果您想更清楚地知道应该启用什么功能，应该搜索术语SFINAE，这是一种模板元编程技术，用于确定函数或类是否应该存在。
In the <> brackets you see the compile-time arguments or more accurately the template parameters, and the function in this case takes 0 runtime arguments in the () brackets. The last compile time argument has a default argument which is used to check whether the function should compile at all (enable_if type). If you want to know more clearly what enable if does you should search for the term SFINAE, which is a template metaprogramming technique used to determine whether a function or class should exist or not.
这是SFINAE的简短示例：
Here is a short SFINAE example:
template<typename T, typename = std::enable_if_t<std::is_floating_point<T>::value>>
void function(T arg)
{
}
function(0.3f); //OK
function(0.0); //OK double results in std::is_floating_point<double>::value == true
function("Hello"); //Does not exist (T is not floating point)
第三个函数调用失败的原因是因为该功能不存在。这是因为当作为模板参数传入的编译时布尔值为false时，启用if导致该函数不存在。
The reason the third function call fails, is because the function does not exist. This is because the enable if caused the function not to exist when the compile-time bool that is passed in as its' template argument is false.
std::is_floating_point<std::string>::value == false
Do请注意，很多人都认为SFINAE语法太可怕了，并且随着C ++ 20概念和约束的引入，不再需要很多SFINAE代码。
Do note that a lot of people agree that the SFINAE syntax is horrible and that a lot of SFINAE code will not be necessary anymore with the introduction of concepts and constraints in C++ 20.
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