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<main>
<h1 id="use-constexpr"><a class="header" href="constexpr.html#use-constexpr">use constexpr</a></h1>
<p>used to create compilation time constants which are useful in template params, array sizes etc. <code>true constants</code></p>
<p>a variable or a function can be defined as a constexpr and their values will be set at compilation time if possible or they can't be used as true constants.</p>
<blockquote>
<p>constexpr => const + expr => an adjective</p>
</blockquote>
<p><strong>what's expr?</strong></p>
<blockquote>
<p>expression means something yields a result !</p>
</blockquote>
<pre><code class="language-c++">
// arithmetic ops
auto result = 5 + 4;
// logical ops
bool isEqual = (5 == 5);
// assignment ops
int x = 10;
// function calls
int sum = call(1,2);
</code></pre>
<pre><code class="language-c++">constexpr auto result = 5 + 4;
static_assert(result == 9);
constexpr auto eq = (5 == 5);
static_assert(eq);
constexpr int x = 10;
static_assert( x == 10);
constexpr auto call(int x, int y) { return x + y; }
constexpr auto sum = call(1, 4);
static_assert( sum == 5);
</code></pre>
<blockquote>
<p>using constexpr doesn't guarantee that your expression will be evaluated at compilation time</p>
</blockquote>
<blockquote>
<p>heap types are invalid for constexpr like heap-pointers, std::string, std::vector etc...</p>
</blockquote>
<blockquote>
<p>{std::string, std::vector} has some specific cases (if size < N then use stack mem) which allows constexpr</p>
</blockquote>
<pre><code class="language-c++">
constexpr std::vector<std::string_view> split_sv(std::string_view strv, std::string_view delims = " ") {
std::vector<std::string_view> output;
size_t first = 0;
while (frist < strv.size()) {
const auto second = strv.find_first_of(delims, first);
if (first != second) {
output.emplace_back(strv.substr(first, second - first));
}
if (second == std::string_view::npos) brea;
first = second + 1;
}
return output;
}
constexpr const char* brk(const char* ptr, const char* delims) {
while (*ptr != '\0') {
const char* it = delims;
while (*it != '\0') {
if (*it == *ptr) return ++ptr;
it++;
}
ptr++;
}
return ptr;
}
constexpr size_t num_of_words(const char* ptr, const char* delims = " ") {
size_t count = 0;
do {
ptr = brk(ptr, delims);
if (*ptr != '\0') count++;
} while(*ptr != '\0');
return ++count;
}
constexpr size_t num_of_words(std::string_view str) {
const auto words = split_sv(str);
return words.size();
}
</code></pre>
<blockquote>
<p>more examples...</p>
</blockquote>
<pre><code class="language-c++">
constexpr auto calc(const std::array<int, 10>& data) {
auto total = 0;
for (auto v : data) total += v;
return total;
}
int main() {
constexpr std::array<int, 10> arr = {1, 2, 3, 4, 5};
constexpr auto total = calc(arr);
static_assert(total == 15);
return 0;
}
</code></pre>
<blockquote>
<p>more examples...</p>
</blockquote>
<pre><code class="language-c++">
struct S {
constexpr S(int value): m_value(value) {} // normal
constexpr int getValue() const noexcept { return 100 * m_value; }
void setValue(int value) { m_value = value; } // extra and not included in constexprs...
int m_value;
};
int main() {
S s{100}; // => can call all
constexpr S s {100}; // => cannot call getValue() => breaks the rule of compilation time calculations
return 0;
}
</code></pre>
<blockquote>
<p>more examples...</p>
</blockquote>
<pre><code class="language-c++">
template <typename T>
constexpr auto absolute(T arg) {
return arg < 0 ? -arg : arg;
}
template <typename T>
constexpr auto epsilon = T(0.000001);
template <typename T>
constexpr auto close_enough(T a, T b) {
if constexpr (std::is_floating_point_v<T>)
return absolute(a - b) < epsilon<T>;
else
return a == b;
}
int main() {
constexpr auto same = close_enough(1, 2);
static_assert(!same);
}
</code></pre>
<blockquote>
<p>more examples...</p>
</blockquote>
<pre><code class="language-c++">
template <typename T>
constexpr auto absolute(T arg) {
return arg < 0 ? -arg : arg;
}
template <typename T>
constexpr auto precision = T(0.00001);
template <typename T>
requires std::is_floating_point_v<T>
constexpr auto close_enough(T a, T b) {
return absolute(a - b) < precision<T>;
}
constexpr auto close_enough(auto a, auto b) { return a == b; }
int main() {
constexpr auto r1 = close_enough(1, 2);
constexpr auto r2 = close_enough(1.2f, 2.3f);
}
</code></pre>
<h2 id="constexpr-vs-macro"><a class="header" href="constexpr.html#constexpr-vs-macro">constexpr vs macro</a></h2>
<blockquote>
<p>Don't use macros for creating variables or doing some simple logical ops</p>
</blockquote>
<blockquote>
<p>Macro is good for boiler plating... but other cases can be handled with constexpr easily</p>
</blockquote>
<pre><code class="language-c++">
#define M(X) (X * X)
constexpr auto MX(size_t N) { return N * N; }
template <size_t N>
constexpr auto MXV = N * N;
int main() {
constexpr auto x = M(10);
static_assert(x == 100);
constexpr auto x2 = MX(10);
static_assert(x2 == 100);
constexpr auto x3 = MXV<10>;
static_assert(x3 == 100);
}
</code></pre>
<blockquote>
<p>more examples...</p>
</blockquote>
<pre><code class="language-c++">
constexpr auto cala(std::size_t value) { return value * 10; }
constexpr auto calb(bool value) { return value ? "hakan" : "gedek"; }
constexpr auto calc(const char* name) {
if (name == "hakan")
return name;
else if (name == "gedek")
return name;
else
return "unknown";
}
constexpr auto cald(std::initializer_list<int> l) {
auto total = 0;
for (auto item : l) total += item;
return total;
}
constexpr auto cale(const size_t param) {
if (param == 100)
return param;
else
return size_t(0);
}
int main() {
constexpr auto r1 = cala(1);
static_assert(r1 == 10);
constexpr auto r2 = calb(false);
static_assert(r2 == "gedek");
constexpr auto r3 = calc("gedek");
static_assert(r3 == "gedek");
constexpr auto r4 = cald({1, 2, 3});
static_assert(r4 == 6);
const size_t p1 = 100;
constexpr auto r5 = cale(p1);
static_assert(r5 == p1);
}
</code></pre>
<blockquote>
<p>more examples...</p>
</blockquote>
<pre><code class="language-c++">// normal constexpr function
constexpr auto calc(int p1, int p2) { return p1 + p2; }
constexpr auto calc(std::initializer_list<int> l) {
return std::accumulate(l.begin(), l.end(), 0);
}
int main() {
const auto r = calc(1, 2);
const auto r2 = calc({1, 2, 3});
constexpr std::initializer_list<int> l {1, 2, 3}; // this fails
constexpr auto r3 = calc(l);
static_assert(r == 3);
static_assert(r2 == 6);
static_assert(r3 == 6); // This fails
}
</code></pre>
<blockquote>
<p>more examples...</p>
</blockquote>
<pre><code class="language-c++">
// normal template constexpr variable
template <size_t N>
constexpr auto calc2(int p1, int p2) {
return N * p1 + N * p2;
}
template <typename Cont = std::vector<int>>
constexpr auto calc2(Cont&& cont) {
return std::accumulate(cont.begin(), cont.end(), 0);
}
int main() {
const auto r = calc2<10>(1, 2);
static_assert(r == 30);
const auto r2 = calc2({1, 2, 3});
static_assert(r2 == 6);
// Not works =. constexpr will show that this ctor call will trigger new operator so it fails
constexpr std::vector<int> v = {1, 2, 3};
const auto r3 = calc2(v);
static_assert(r3 == 6);
}
</code></pre>
<blockquote>
<p>variables...</p>
</blockquote>
<pre><code class="language-c++">template <size_t N>
constexpr auto Fib = Fib<N - 1> + Fib<N - 2>;
template <>
constexpr auto Fib<0> = 0;
template <>
constexpr auto Fib<1> = 1;
</code></pre>
<pre><code class="language-c++">// normal template constexpr variable
template <size_t N>
constexpr auto X2 = N * N;
template <size_t N>
constexpr auto X3 = N * N * N;
template <size_t N>
constexpr auto X2Div10 = N * N / 10;
int main() {
static_assert(X2<10> == 100);
static_assert(X3<3> == 27);
static_assert(X2Div10<10> == 10);
}
</code></pre>
<blockquote>
<p>a custom type</p>
</blockquote>
<pre><code class="language-c++">
template <class T, class E>
struct expected {
std::variant<T, E> data;
constexpr expected(T value): data{std::move(value)} {}
constexpr expected(E err): data{std::move(err)} {}
constexpr bool is_ok() const { return std::holds_alternative<T>(data); }
constexpr bool is_err() const { return std::holds_alternative<E>(data); }
constexpr std::optional<T> ok() const {
if(is_ok()) return std::get<T>(data);
return std::nullopt;
}
constexpr std::optional<E> err() const {
if (is_err()) return std::get<E>(data);
return std::nullopt;
}
constexpr operator bool() const { return is_ok() ? true : false; }
};
auto parse_number(std::string_view value) -> expected<double, err> {
char const* begin = std.data();
char const* end = nullptr;
double retval = std::strtod(begin, &end);
if (begin == end)
return err::invalid_input;
else if (std::isinf(retval))
return err::overflow;
value.remove_prefix(end - begin);
return retval;
}
int main() {
for (auto src: {"31"sv, "123hakan"sv, "hakan"sv})
{
auto result = parse_number(src);
std::println("{}", result.ok().or_else([]{ return std::optional<double>{-1}}). value());
}
}
</code></pre>
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