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<h1 id="evaluate-constness"><a class="header" href="evaluate_const.html#evaluate-constness">evaluate constness</a></h1>
<blockquote>
<p>using const in function interface helps ?</p>
</blockquote>
<p>Yes. You inform the compiler; this temp variable will not be updated</p>
<pre><code class="language-c++">
void foo(const int value) {} // only read
void bar(int value) {
value++
} // read and write
</code></pre>
<blockquote>
<p>Can const be hidden in T in templates ?</p>
</blockquote>
<p>Yes. Always use decay_t to access correct T ( raw T) .</p>
<p>T can be:</p>
<pre><code class="language-c++">
int i = 0;
const int ci = 0;
int& ri = i;
const int& cri = ci;
0
void foo(T&&) [with T = int&]
std::is_lvalue_reference_v<T>: 1
std::is_lvalue_reference_v<decltype(param)>: 1
std::is_rvalue_reference_v<T>: 0
std::is_rvalue_reference_v<decltype(param)>: 0
void foo(T&&) [with T = const int&]
std::is_lvalue_reference_v<T>: 1
std::is_lvalue_reference_v<decltype(param)>: 1
std::is_rvalue_reference_v<T>: 0
std::is_rvalue_reference_v<decltype(param)>: 0
void foo(T&&) [with T = int&]
std::is_lvalue_reference_v<T>: 1
std::is_lvalue_reference_v<decltype(param)>: 1
std::is_rvalue_reference_v<T>: 0
std::is_rvalue_reference_v<decltype(param)>: 0
void foo(T&&) [with T = const int&]
std::is_lvalue_reference_v<T>: 1
std::is_lvalue_reference_v<decltype(param)>: 1
std::is_rvalue_reference_v<T>: 0
std::is_rvalue_reference_v<decltype(param)>: 0
void foo(T&&) [with T = int]
std::is_lvalue_reference_v<T>: 0
std::is_lvalue_reference_v<decltype(param)>: 0
std::is_rvalue_reference_v<T>: 0
std::is_rvalue_reference_v<decltype(param)>: 1
</code></pre>
<p>if you check T => you cannot access correct type alone...use std::forward<T&&> with it or you can use decltype(param) => this always carry correct type</p>
<pre><code class="language-c++">
void foo(const T&) [with T = int]
std::is_lvalue_reference_v<T>: 0
std::is_lvalue_reference_v<decltype(param)>: 1
std::is_rvalue_reference_v<T>: 0
std::is_rvalue_reference_v<decltype(param)>: 0
void foo(const T&) [with T = int]
std::is_lvalue_reference_v<T>: 0
std::is_lvalue_reference_v<decltype(param)>: 1
std::is_rvalue_reference_v<T>: 0
std::is_rvalue_reference_v<decltype(param)>: 0
void foo(const T&) [with T = int]
std::is_lvalue_reference_v<T>: 0
std::is_lvalue_reference_v<decltype(param)>: 1
std::is_rvalue_reference_v<T>: 0
std::is_rvalue_reference_v<decltype(param)>: 0
void foo(const T&) [with T = int]
std::is_lvalue_reference_v<T>: 0
std::is_lvalue_reference_v<decltype(param)>: 1
std::is_rvalue_reference_v<T>: 0
std::is_rvalue_reference_v<decltype(param)>: 0
void foo(const T&) [with T = int]
std::is_lvalue_reference_v<T>: 0
std::is_lvalue_reference_v<decltype(param)>: 1
std::is_rvalue_reference_v<T>: 0
std::is_rvalue_reference_v<decltype(param)>: 0
</code></pre>
<blockquote>
<p>Can const be hidden in auto ?</p>
</blockquote>
<p>auto can be T or T* but adjectives like & or const you need to use them separately.</p>
<p>auto => int
auto => int*
const auto => const int
const auto => const int*
auto& => int &
auto& => int*&
const auto& => const int &
const auto& => const int*&</p>
<p>auto&& => all upper ones or int&& ...</p>
</main>
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