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<h1 id="c"><a class="header" href="#c">c++</a></h1>
<h2 id="type-deduction"><a class="header" href="#type-deduction">Type deduction</a></h2>
<p>Force compile error to see what <code>auto</code> is deduced to.</p>
<pre><code class="language-cpp">auto foo = bar();
// force compile error
typename decltype(foo)::_;
</code></pre>
<h2 id="variadic-templates-parameter-pack"><a class="header" href="#variadic-templates-parameter-pack">Variadic templates (<a href="https://en.cppreference.com/w/cpp/language/parameter_pack">parameter pack</a>)</a></h2>
<pre><code class="language-cpp">#include <iostream>
// -- Example 1 - print template value arguments.
// Base case with one parameter.
template<int P>
void show_int() {
printf("%d\n", P);
}
// General case with at least two parameters, to disambiguate from base case.
template<int P0, int P1, int... Params>
void show_int() {
printf("%d, ", P0);
show_int<P1, Params...>();
}
// -- Example 2 - print values of different types.
// Base case with one parameter.
template<typename T>
void show(const T& t) {
std::cout << t << '\n';
}
// General case with at least two parameters, to disambiguate from base case.
template<typename T0, typename T1, typename... Types>
void show(const T0& t0, const T1& t1, const Types&... types) {
std::cout << t0 << ", ";
show(t1, types...);
}
int main() {
show_int<1, 2, 3, 4, 5>();
show(1, 1.0, "foo", 'a');
}
</code></pre>
<h2 id="example-any_of-template-meta-function"><a class="header" href="#example-any_of-template-meta-function">Example: <code>any_of</code> template meta function</a></h2>
<pre><code class="language-cpp">#include <type_traits>
template<typename T, typename... U>
struct any_of : std::false_type {};
// Found our type T in the list of types U.
template<typename T, typename... U>
struct any_of<T, T, U...> : std::true_type {};
// Pop off the first element in the list of types U,
// since it didn't match our type T.
template<typename T, typename U0, typename... U>
struct any_of<T, U0, U...> : any_of<T, U...> {};
// Convenience template variable to invoke meta function.
template<typename T, typename... U>
constexpr bool any_of_v = any_of<T, U...>::value;
static_assert(any_of_v<int, char, bool, int>, "");
static_assert(!any_of_v<int, char, bool, float>, "");
</code></pre>
<h2 id="example-sfinae-enable_if"><a class="header" href="#example-sfinae-enable_if">Example: <a href="https://en.cppreference.com/w/cpp/language/sfinae">SFINAE</a> (<a href="https://en.cppreference.com/w/cpp/types/enable_if">enable_if</a>)</a></h2>
<p>Provide a single entry point <code>Invoke</code> to call some <code>Operations</code>.
Use <code>enable_if</code> to enable/disable the template functions depending on the two
available traits an operation can have:</p>
<ul>
<li>Operation returns a result</li>
<li>Operation requires a context</li>
</ul>
<pre><code class="language-cpp">#include <iostream>
#include <type_traits>
// Helper meta fns.
template<typename T>
using enable_if_bool = std::enable_if_t<T::value, bool>;
template<typename T>
using disable_if_bool = std::enable_if_t<!T::value, bool>;
template<typename T>
using has_dst = std::integral_constant<bool, !std::is_same<typename T::Return, void>::value>;
// Template meta programming invoke machinery.
namespace impl {
// Invoke an OPERATION which *USES* a context.
template<typename Ctx, template<typename> class Op, typename... P,
enable_if_bool<typename Op<Ctx>::HasCtx> = true>
typename Op<Ctx>::Return Invoke(const Ctx& C, P... params) {
return Op<Ctx>()(C, params...);
}
// Invoke an OPERATION which uses *NO* context.
template<typename Ctx, template<typename> class Op, typename... P,
disable_if_bool<typename Op<Ctx>::HasCtx> = true>
typename Op<Ctx>::Return Invoke(const Ctx&, P... params) {
return Op<Ctx>()(params...);
}
} // namespace impl
// Invoke an OPERATION which *HAS* a DESTINATION with arbitrary number of arguments.
template<typename Ctx, template<typename> class Op, typename... P,
enable_if_bool<has_dst<Op<Ctx>>> = true>
void Invoke(const Ctx& C, P... params) {
std::cout << "Invoke " << Op<Ctx>::Name << '\n';
typename Op<Ctx>::Return R = impl::Invoke<Ctx, Op>(C, params...);
std::cout << "returned -> " << R << '\n';
}
// Invoke an OPERATION which has *NOT* a DESTINATION with arbitrary number of arguments.
template<typename Ctx, template<typename> class Op, typename... P,
disable_if_bool<has_dst<Op<Ctx>>> = true>
void Invoke(const Ctx& C, P... params) {
std::cout << "Invoke " << Op<Ctx>::Name << " without destination." << '\n';
impl::Invoke<Ctx, Op>(C, params...);
}
// Custom context.
struct Ctx {
void out(const char* s, unsigned v) const { printf("%s%x\n", s, v); }
};
// Operations to invoke.
template<typename Ctx>
struct OpA {
using HasCtx = std::false_type;
using Return = int;
static constexpr const char* const Name = "OpA";
constexpr Return operator()(int a, int b) const { return a + b; }
};
template<typename Ctx>
struct OpB {
using HasCtx = std::true_type;
using Return = void;
static constexpr const char* const Name = "OpB";
Return operator()(const Ctx& C, unsigned a) const { C.out("a = ", a); }
};
int main() {
Ctx C;
Invoke<Ctx, OpA>(C, 1, 2);
Invoke<Ctx, OpB>(C, 0xf00du);
return 0;
}
</code></pre>
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