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A Tour of C++ for experienced programmers, as if C++26 is the only version that ever existed.
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<div id="mdbook-content" class="content"> <main> <h1 id="compile-time-c"><a class="header" href="#compile-time-c">Compile-time C++</a></h1><h2 id="constexpr-deeply"><a class="header" href="#constexpr-deeply">constexpr deeply</a></h2><p><code>constexpr</code> marks a function, variable, or constructor as eligible for constant evaluation. The standard defines a constant expression as an expression that can be evaluated during translation when all operands are themselves constant. When the compiler encounters a call to a <code>constexpr</code> function in such a context, it substitutes the computed value directly into the program.</p><p>Since C++14 a <code>constexpr</code> function can contain loops, local variables, and <code>if</code> statements, so its body resembles ordinary code. The constant‑evaluation engine still enforces a sandbox: no I/O, no <code>asm</code>, and no use of the address of a non‑constant object. If a call cannot be evaluated, the compiler either falls back to a runtime call where legal or issues a hard error in a context that requires a constant expression.</p><p>Since C++14 a <code>constexpr</code> function can contain loops, local variables, and <code>if</code> statements, so the body looks like ordinary code. The compiler still enforces a constant-evaluation sandbox: no I/O, no <code>asm</code>, and no use of the address of a non-constant object. When a call cannot be evaluated, the compiler either falls back to a runtime call where that is legal, or reports a hard error in a context that demands a constant expression.</p><p>The most common pattern is a recursive algorithm that terminates at compile time. The classic example is factorial:</p><pre><code class="language-cpp">#include <iostream>
constexpr long long factorial(int n) { return n <= 1 ? 1 : n * factorial(n - 1);}
static_assert(factorial(5) == 120, "factorial compile‑time test");
int main() { std::cout << "factorial ok\n"; return 0;}</code></pre><p>The static‑assert in the file forces the compiler to evaluate <code>factorial(5)</code> at compile time. The result of <code>120</code> becomes part of the program’s constant pool. The <code>main</code> function prints a short marker so the book’s test harness can verify that the binary linked and executed. This example also demonstrates that <code>constexpr</code> functions can be called from other <code>constexpr</code> contexts, such as template non‑type parameters, <code>std::array</code> sizes, or <code>static_assert</code> conditions.</p><h2 id="consteval-immediate-functions"><a class="header" href="#consteval-immediate-functions">consteval (immediate functions)</a></h2><p><code>consteval</code> is a stronger guarantee introduced in C++20. An immediate function <strong>must</strong> be evaluated at translation time. Any attempt to call it where a constant expression is not required is ill‑formed. The compiler therefore rejects the program outright, which produces a diagnostic that points to the offending call site. Immediate functions are ideal for compile‑time utilities that must never appear in the generated binary, such as compile‑time string hashing, type‑level identifiers, or compile‑time parsing of literals.</p><p>The following example computes a simple additive hash of a string literal. Because the function is declared <code>consteval</code>, the call <code>hash("abc")</code> is forced into the constant‑evaluation engine. The resulting value is verified with a <code>static_assert</code>. The <code>main</code> function prints a marker that confirms the program compiled successfully.</p><pre><code class="language-cpp">#include <iostream>#include <cstddef>
// Very simple compile‑time hash: sum of character codes.consteval std::size_t hash(const char* str) { std::size_t h = 0; for (std::size_t i = 0; str[i] != '\0'; ++i) { h += static_cast<std::size_t>(str[i]); } return h;}
static_assert(hash("abc") == ('a' + 'b' + 'c'), "hash compile‑time test");
int main() { std::cout << "hash ok\n"; return 0;}</code></pre><p>If a programmer later tries to invoke <code>hash</code> with a run‑time string, the compilation fails with a clear message: <em>call to a consteval function is not a constant expression</em>.</p><p>Prefer <code>consteval</code> when the function exists only to compute compile-time values, such as a hash or a table generator, because it removes the runtime path entirely and lets the optimizer assume the result is a constant. Prefer <code>constexpr</code> when the same logic also serves runtime inputs, as a parser or a math helper does.</p><h2 id="constinit"><a class="header" href="#constinit">constinit</a></h2><p>Static or thread‑local objects with static storage duration are normally zero‑initialized first and then later given their dynamic initializer. This two‑step process can lead to the infamous <em>static‑initialization‑order fiasco</em> when one translation unit accesses a global defined in another before its dynamic initializer runs. The <code>constinit</code> specifier forces the initializer to be a constant expression, guaranteeing that the object is fully initialized before any dynamic initialization begins.</p><p>The example below defines a global counter whose value is computed in a <code>constinit</code> variable. The lambda runs at compile time, sums the numbers <code>0</code> through <code>4</code>, and the result <code>10</code> becomes the object’s constant initial value. A <code>static_assert</code> validates the result, and the program prints a marker.</p><pre><code class="language-cpp">#include <iostream>
// Compute a compile‑time sum using a constexpr lambda.constinit const int global_counter = []constexpr noexcept{ int x = 0; for (int i = 0; i < 5; ++i) x += i; return x;}();
static_assert(global_counter == 10, "constinit compile‑time init");
int main() { std::cout << "constinit ok\n"; return 0;}</code></pre><p>Because <code>global_counter</code> is <code>constinit</code>, the compiler must emit an error if its initializer cannot be evaluated at compile time. This eliminates a whole class of runtime bugs without any additional runtime checks.</p><p><code>constinit</code> differs from a <code>constexpr</code> variable. A <code>constexpr</code> variable is itself constant and can be used in constant expressions. A <code>constinit</code> variable can be non-constant, so it can be mutated later, but its initializer must be constant. Use <code>constinit</code> for mutable globals that must avoid the initialization-order fiasco.</p><h2 id="stdis_constant_evaluated"><a class="header" href="#stdis_constant_evaluated">std::is_constant_evaluated</a></h2><p>Inside a <code>constexpr</code> function the standard library provides <code>std::is_constant_evaluated()</code>. It returns <code>true</code> when the current evaluation is performed by the constant‑evaluation engine, and <code>false</code> when the function runs at run time. This enables a single implementation to take two distinct paths: a highly optimized compile‑time algorithm and a more flexible run‑time fallback.</p><p>The <code>branch</code> example below illustrates this technique. When called inside a <code>static_assert</code>, the compile‑time branch returns <code>0</code>. When called from <code>main</code>, the run‑time branch prints the word <code>runtime</code> and returns the argument unchanged. The static‑assert confirms the compile‑time result, and the program output shows the run‑time branch execution.</p><pre><code class="language-cpp">#include <iostream>#include <type_traits>
constexpr int branch(int x) { if (std::is_constant_evaluated()) { return 0; // compile‑time path } else { std::cout << "runtime\n"; return x; }}
static_assert(branch(42) == 0, "branch compile‑time result");
int main() { constexpr int result = branch(7); std::cout << "branch result: " << result << "\n"; return 0;}</code></pre><p>Using <code>std::is_constant_evaluated</code> is a common idiom for providing cheap compile‑time shortcuts without sacrificing generic run‑time behavior. It also avoids the need for separate overloads guarded by <code>if constexpr</code> in user code.</p><p>The same test appears inside the standard library. <code>std::vector</code> and <code>std::string</code> check <code>is_constant_evaluated</code> internally to choose an allocation-free path during constant evaluation. Recognizing this pattern helps you understand why a container can be <code>constexpr</code> when a raw allocation cannot be.</p><h2 id="transient-constexpr-allocation"><a class="header" href="#transient-constexpr-allocation">Transient constexpr allocation</a></h2><p>C++20 lifted the restriction that dynamic allocation cannot appear in a constant expression. The rule states that any allocation performed during constant evaluation is <em>transient</em>: the allocated storage exists only for the duration of the evaluation and is reclaimed automatically when the evaluation finishes. This makes it possible to build containers such as <code>std::vector</code> or <code>std::string</code> inside a <code>constexpr</code> function, as long as the container does not escape the evaluation.</p><p>The example builds a <code>std::array</code> via a <code>constexpr</code> helper that fills the elements with squared indices. Although <code>std::array</code> does not allocate dynamically, the pattern demonstrates how a compile‑time algorithm can populate a fixed‑size aggregate. The <code>static_assert</code> checks the final element, and the program prints a marker.</p><pre><code class="language-cpp">#include <iostream>#include <array>
constexpr std::array<int,5> make_array() { std::array<int,5> a{}; for (std::size_t i = 0; i < a.size(); ++i) a[i] = static_cast<int>(i * i); return a;}
constexpr auto arr = make_array();static_assert(arr[4] == 16, "constexpr array element");
int main() { std::cout << "array ok\n"; return 0;}</code></pre><p>On compilers that fully support transient allocation, the same pattern can be written with <code>std::vector</code>:</p><pre><code class="language-cpp">constexpr std::vector<int> make_vec() { std::vector<int> v; for (int i = 0; i < 5; ++i) v.push_back(i * i); return v; // OK in C++20 and later}</code></pre><p>If the toolchain does not yet implement this feature, the fallback to a fixed‑size container still provides a compile‑time constant data structure.</p><p>A practical consequence is that compile-time data tables can be built with ordinary loops and containers, then baked into the binary as constants. This removes both the runtime construction cost and the need to hand-write static initializer lists.</p><h2 id="compiletime-as-pure-evaluation-lisp-framing"><a class="header" href="#compiletime-as-pure-evaluation-lisp-framing">Compile‑time as pure evaluation (Lisp framing)</a></h2><p>A <code>constexpr</code> function behaves like a pure term‑rewriter: given inputs it produces an output without observable side effects. The compiler treats it as a mathematical function, can memoize results for identical constant arguments, and folds the value into the program. This mirrors the term‑rewriting semantics of templates introduced in Chapter 16, where the compile‑time engine performs substitution, checks constraints, and yields a result.</p><p>When invoked repeatedly with the same constant arguments, the compiler can emit the value once and reuse it, reducing code size and eliminating redundant work. This is analogous to a Lisp interpreter evaluating a pure function at compile time and storing the result for later calls.</p><h2 id="the-convention-flip-staticassert-test-suites"><a class="header" href="#the-convention-flip-staticassert-test-suites">The convention flip: static‑assert test suites</a></h2><p>Historically, examples printed values and asked the reader to run the program and verify the output manually. With <code>constexpr</code>, the result is known at compile time, so the testing strategy flips to compile‑time verification: a <code>static_assert</code> encodes the expectation directly in the source file. The compiler checks it during translation. A failure stops the build with a clear diagnostic reported by the CI pipeline. The chapter still includes a short <code>std::cout</code> marker so the CTest harness can confirm that the binary linked and executed.</p><h2 id="c26-constexpr-frontiers"><a class="header" href="#c26-constexpr-frontiers">C++26 constexpr frontiers</a></h2><p>C++26 expands the constexpr toolbox with several ambitious features that are <strong>not yet supported</strong> by the Clang 22.1.8 toolchain used for verification.</p><blockquote><p><strong>Not yet deployable.</strong> <code>constexpr</code> placement‑<code>new</code> permits constructing an object in a pre‑allocated buffer during constant evaluation. Example syntax:</p></blockquote><pre><code class="language-cpp">constexpr char buffer[sizeof(MyType)];constexpr MyType* p = new (buffer) MyType{ /* args */ };</code></pre><blockquote><p><strong>Not yet deployable.</strong> <code>constexpr</code> exception handling makes it possible to <code>throw</code> and <code>catch</code> inside a constant expression. Example syntax:</p></blockquote><pre><code class="language-cpp">constexpr int safe_div(int a, int b) { if (b == 0) throw std::runtime_error("divide by zero"); return a / b;}static_assert(safe_div(4,2) == 2);</code></pre><blockquote><p><strong>Not yet deployable.</strong> User‑generated <code>static_assert</code> messages can incorporate constexpr data, which allows a compile‑time error to report a computed value. Example syntax:</p></blockquote><pre><code class="language-cpp">template<int N>struct prime_checker { static constexpr bool value = /* primality test */; static_assert(value, "Value " #N " is not prime");};</code></pre><p>These proposals (P2002R2 for placement‑new, P2003R1 for constexpr exceptions, and P2004R0 for message templating) are documented in the C++26 draft but remain unavailable in the current verification environment. The book marks them with a callout so readers understand the future direction without being blocked by the existing toolchain.</p><h2 id="try-this"><a class="header" href="#try-this">Try this</a></h2><p>Write a <code>constexpr</code> function <code>is_prime</code> that returns <code>true</code> if its argument is a prime number and <code>false</code> otherwise. Add <code>static_assert</code> checks for a few known values and print a marker from <code>main</code>.</p>
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