[英]Select a variable at compile-time
I'd like to be able to specify via a boolean which of two variables I need to use at compile time, all of this without direct SFINAE . 我希望能够通过布尔指定我需要在编译时使用哪两个变量,所有这些都没有直接的SFINAE 。 Just one single function, similar to
std::conditional
but not returning types. 只有一个函数,类似于
std::conditional
但不返回类型。
So for instance in a class test
, I'd like to have 因此,例如在课堂
test
,我想拥有
class test
{
template <class T, bool first, MAGIC_HAPPENS var>
void apply_it(T &t)
{
for (auto &p : var) p.apply(t);
}
std::vector<myfunction> var1;
std::vector<myfunction> var2;
}
The use is simple: if I specify first == true
then it should apply the loop with var == var1
, otherwise var == var2
. 使用很简单:如果我指定
first == true
那么它应该应用var == var1
的循环,否则var == var2
。
Is it possible? 可能吗?
Just for fun(*), a minimal C++17(**) modification to your current code snippet might be 只是为了好玩(*),对当前代码片段的最小 C ++ 17(**)修改可能是
class test
{
std::vector<myfunction> var1;
std::vector<myfunction> var2;
template <class T, bool first, std::vector<myfunction> test::*var = first ? &test::var1 : &test::var2 >
void apply_it(T &t)
{
for (auto &p : this->*var) p.apply(t);
}
};
(*) I see no situation in which such a thing would be preferable over the other suggested solutions ... (*)我认为没有哪种情况比其他建议的解决方案更可取...
(**) as far as I know, this requires C++17 due to linkage requirement of template non type pointer parameters ... (**)据我所知,由于模板非类型指针参数的链接要求,这需要C ++ 17 ...
For C++17 and above: 对于C ++ 17及以上版本:
class test
{
template <class T, bool first>
void apply_it(T &t)
{
if constexpr (first)
{
for (auto &p : var1) p.apply(t);
}
else
{
for (auto &p : var2) p.apply(t);
}
}
std::vector<myfunction> var1;
std::vector<myfunction> var2;
}
if constexpr
is evaluated at compile time if the condition is constexpr, which is the case for template parameters. if constexpr
在编译时被评估,如果条件是constexpr,则模板参数就是这种情况。
You can do that in C++11 with a pointer to data member: 您可以在C ++ 11中使用指向数据成员的指针执行此操作:
class test
{
template <class T, bool first>
void apply_it(T &t)
{
constexpr auto var = first ? &test::var1 : &test::var2;
for (auto &p : this->*var) p.apply(t);
}
std::vector<myfunction> var1;
std::vector<myfunction> var2;
}
template<std::size_t I, class...Args>
decltype(auto) pick( Args&&... args ) {
return std::get<I>( std::forward_as_tuple( std::forward<Args>(args)... ) );
}
pick
selects at compile time something from a list. pick
在编译时从列表中选择一些东西。
class test
{
template <bool first, class T>
void apply_it(T&& t)
{
auto&& var = pick<first?0:1>(var1, var2);
for (auto&& p : var)
p.apply(t);
}
std::vector<myfunction> var1;
std::vector<myfunction> var2;
};
I also made some minor improvements while I was copy pasting. 我复制粘贴时也做了一些小的改进。
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