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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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<h1 class="menu-title">A Tour of C++26</h1>
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<div id="mdbook-content" class="content"> <main> <h1 id="errors-and-contracts"><a class="header" href="#errors-and-contracts">Errors and contracts</a></h1><h2 id="the-c-legacy"><a class="header" href="#the-c-legacy">The C legacy</a></h2><p>C reports failure via integer return codes and the global variable <code>errno</code>. Callers must remember to test each result, otherwise silent bugs arise.</p><p>C++ adds mechanisms that make ignoring failures harder: the type system can encode error states, and the <code>[[nodiscard]]</code> attribute warns when a result is discarded, ensuring the intent to handle errors is visible.</p><h2 id="exceptions-and-unwinding"><a class="header" href="#exceptions-and-unwinding">Exceptions and unwinding</a></h2><p>C++ supports <code>throw</code>, <code>try</code>, and <code>catch</code>. A <code>throw</code> aborts the current function, triggers stack unwinding, and destroys each local object in reverse order, allowing RAII objects to release resources automatically and preventing leaks.</p><p>Exceptions apply when the failure cannot be repaired at the call site, such as out‑of‑memory, corrupted files, or invalid user input.</p><p>The dividing line between exceptions and <code>expected</code> is the frequency and the locality of the failure. A parse error is routine: the caller expects it, handles it, and moves on, so it travels as an <code>expected</code> value. An out-of-memory error is rare and crosses abstraction boundaries: no individual caller can fix it, so it travels as an exception and unwinds to the nearest handler. Mixing the two is a smell: if every caller wraps a function in a <code>try</code>/<code>catch</code>, the failure is routine and belongs in an <code>expected</code>.</p><p>Modern ABIs place the exception handling code on the cold path only. The common case executes without extra branches or hidden calls. The claim that exceptions add overhead no longer applies to the common case. When exceptions are disabled with <code>-fno-exceptions</code>, the compiler omits unwind tables, reducing binary size and improving load time.</p><h2 id="noexcept"><a class="header" href="#noexcept"><code>noexcept</code></a></h2><p>The specifier <code>noexcept</code> promises that a function will not throw. If a <code>noexcept</code> function does throw, the runtime calls <code>std::terminate</code>.</p><p><code>noexcept</code> is a contract, not a hint. The compiler uses it to make decisions: a <code>std::vector</code> will move elements during reallocation only if the move constructor is <code>noexcept</code>, and the optimizer can elide exception-handling machinery around a <code>noexcept</code> call. Breaking the contract by throwing from a <code>noexcept</code> function calls <code>std::terminate</code>, which is a hard stop. The specifier is therefore a promise you make when you can guarantee it, not a wish.</p><p>Marking move constructors as <code>noexcept</code> enables the standard library to move objects during vector growth without a fallback copy. The optimizer can also inline the call more aggressively. <code>noexcept</code> functions can be inlined without hidden control flow. The optimizer gains full visibility of the call graph.</p><p>A conditional <code>noexcept(...)</code> expression makes the promise precise: the function is <code>noexcept</code> only when every operation it calls is <code>noexcept</code> itself, so the guarantee evolves with the implementation.</p><pre><code class="language-cpp">#include <iostream>#include <utility>
struct Counter { int value; Counter(int v) : value(v) { std::cout << "constructed " << value << "\n"; } ~Counter() { std::cout << "destroyed " << value << "\n"; } // Move constructor promised not to throw Counter(Counter&& other) noexcept : value(other.value) { std::cout << "move used\n"; // marker for EXPECT other.value = 0; } // Delete copy to emphasize move usage Counter(const Counter&) = delete; Counter& operator=(const Counter&) = delete; Counter& operator=(Counter&& other) noexcept { value = other.value; other.value = 0; return *this; }};
int main(){ Counter a{42}; Counter b = std::move(a); // triggers noexcept move std::cout << "final b=" << b.value << "\n"; return 0;}</code></pre><p>The program prints a marker that shows the <code>noexcept</code> move was used.</p><h2 id="stdexpectedt-e"><a class="header" href="#stdexpectedt-e"><code>std::expected<T, E></code></a></h2><p><code>std::expected<T, E></code> represents either a value of type <code>T</code> or an error of type <code>E</code>. It is a typed, non‑throwing alternative for functions that can fail.</p><p>Compare to <code>std::optional<T></code>: <code>optional</code> conveys only presence or absence. <code>expected</code> also conveys an error description.</p><p>Contrast to exceptions: <code>expected</code> returns an object that the caller must inspect. Control flow stays explicit in the source.</p><pre><code class="language-cpp">#include <iostream>#include <string_view>#include <string>#include <expected>
std::expected<int, std::string> parse_int(std::string_view sv) { try { size_t pos = 0; int value = std::stoi(std::string(sv), &pos); if (pos != sv.size()) return std::unexpected<std::string>("trailing characters"); std::cout << "parsed\n"; // marker for EXPECT return value; } catch (const std::invalid_argument&) { return std::unexpected<std::string>("invalid integer"); } catch (const std::out_of_range&) { return std::unexpected<std::string>("out of range"); }}
int main(){ auto r1 = parse_int("123"); if (r1) { std::cout << "value=" << *r1 << "\n"; } else { std::cout << "error=" << r1.error() << "\n"; } auto r2 = parse_int("abc"); if (r2) { std::cout << "value=" << *r2 << "\n"; } else { std::cout << "error=" << r2.error() << "\n"; } return 0;}</code></pre><p>The demo parses an integer from a string view. On success it prints the value. On error it prints the error string.</p><p><code>std::expected</code> composes. A function that calls another fallible function can pass the error upward with a single expression. <code>.and_then</code> chains success and <code>.or_else</code> handles failure. This keeps the error path linear and avoids the nested <code>if</code>/<code>else</code> pyramids that error-code APIs produce. The type carries both the value and the error, so the compiler tracks whether the result has been checked.</p><p>The monadic interface is what makes <code>expected</code> scale beyond two calls. Without it, a function that calls three fallible operations in a row needs three nested <code>if</code> statements, each checking and forwarding. With <code>.and_then</code>, the same logic is a single chained expression that reads left to right. The error short-circuits: the first failure skips the rest of the chain and returns the error to the caller. This is the same shape as Rust’s <code>?</code> operator, expressed as method calls.</p><h2 id="contracts-c26"><a class="header" href="#contracts-c26">Contracts (C++26)</a></h2><p>C++26 adds four contract attributes that can be placed on statements or function signatures.</p><ul><li><code>[[assert]]</code>: a debugging check that aborts if the condition is false.</li><li><code>[[assume]]</code>: a promise to the optimizer that the condition is always true.</li><li><code>[[expects]]</code>: a precondition that must hold when the function is entered.</li><li><code>[[ensures]]</code>: a postcondition that must hold when the function returns.</li></ul><p>The syntax is inline and looks like a standard attribute.</p><pre><code class="language-cpp">// Illustrative stub: does not compile on all compilers[[expects: a > 0]][[ensures: result >= a ]]int factorial(int a) { [[assert: a >= 0]]; int result = 1; for (int i = 2; i <= a; ++i) result *= i; return result;}</code></pre><p>Compilers differ in support. GCC 16 implements contracts behind a flag. Clang does not yet implement the feature. The snippet therefore serves only as an illustration.</p><p>Contracts encode the same invariants that the earlier chapters express with RAII and lifetimes. When the compiler cannot prove an invariant, a contract can document it and trigger a runtime check in debug builds.</p><p>Contracts and assertions serve different audiences. An assertion checks an internal invariant that the function itself controls. A precondition checks a contract between the caller and the callee: the caller guarantees the condition, and the callee relies on it. A postcondition runs the contract in reverse: the callee guarantees the condition, and the caller relies on it. The four attributes let you state who is responsible for what, which a plain <code>assert</code> cannot express.</p><p>The violation handler decides what happens when a contract breaks. The implementation can ignore the violation, log it, abort, or call a user-defined handler. The default is observe, which logs and continues. The enforce mode aborts. This flexibility lets a project ship with contracts in debug builds and strip them in release, or keep them on in production with a handler that reports to a monitoring service.</p><h3 id="historical-perspective-on-error-handling"><a class="header" href="#historical-perspective-on-error-handling">Historical perspective on error handling</a></h3><p>Early C programs used integer return codes and <code>errno</code> as the sole mechanism for reporting failure. The programmer had to check each call manually, and forgetting a check produced silent corruption. C++98 introduced exceptions as a language feature that separates error propagation from the normal control flow. The first implementations used a table‑based “zero‑cost” model that stored unwind information in the object file. Compilers later added the ability to mark functions <code>noexcept</code>. This ability let the optimizer elide unwind handling for guaranteed‑non‑throwing code. C++23 added <code>std::expected</code> as a standard library type that returns either a value or an error object. Failure became an explicit part of the type system. C++26 formalizes contracts, completing the error‑handling toolbox.</p><h2 id="choosing-a-strategy"><a class="header" href="#choosing-a-strategy">Choosing a strategy</a></h2><div class="table-wrapper"><table><thead><tr><th>Situation</th><th>Recommended tool</th></tr></thead><tbody><tr><td>Failure cannot be recovered locally.</td><td>Throw an exception.</td></tr><tr><td>Failure is routine and the caller must handle it inline.</td><td>Return <code>std::expected</code>.</td></tr><tr><td>Absence carries no extra information.</td><td>Return <code>std::optional</code>.</td></tr><tr><td>Invariant must never be violated.</td><td>Use a contract attribute.</td></tr><tr><td>Function is guaranteed not to throw.</td><td>Mark <code>noexcept</code>.</td></tr></tbody></table></div><p>The table is a decision tree, not a menu. Read the failure mode first, then pick the tool. A function that fails because the input is bad returns <code>expected</code>. A function that fails because the system ran out of memory throws. A function that cannot fail marks itself <code>noexcept</code>. A function that documents a contract uses an attribute. The four tools cover four failure modes, and the modes do not overlap.</p><p>Use the tool that matches the semantic intent and avoid mixing strategies without a clear reason. A library that mixes <code>expected</code> and exceptions forces the caller to hold two mental models. Choose a single default per module and deviate only when the failure profile demands it. For example, a parser returns <code>expected</code> because every call can fail, while a memory allocator throws because out‑of‑memory is rare and unrecoverable.</p><h2 id="impossible-cases"><a class="header" href="#impossible-cases">Impossible cases</a></h2><p>When the program reaches a state that the logic says cannot occur, the code can call one of the following.</p><ul><li><code>std::abort()</code>: terminates the process immediately.</li><li><code>std::terminate()</code>: ends the program after invoking the terminate handler.</li><li><code>std::unreachable()</code>: tells the compiler that this point is unreachable. Reaching it results in undefined behavior.</li></ul><p>These calls are reserved for truly impossible branches such as a default case in a <code>switch</code> that covers every enum value.</p><p><code>std::unreachable</code> is the strongest of the three. It tells the optimizer that the code path is dead, so the compiler can assume it never runs and use that fact to improve the surrounding code. If the assumption is wrong, the behaviour is undefined. Use it only after a construct that exhausts every case, such as a <code>switch</code> over a <code>std::variant</code> where the visitor covers every alternative. <code>std::terminate</code> is the safe fallback when the program is in a state you cannot reason about: it stops without running destructors, which avoids making things worse.</p><h2 id="try-this"><a class="header" href="#try-this">Try this</a></h2><p>Write a function</p><pre><code class="language-cpp">std::expected<double, std::string> divide(double a, double b);</code></pre><p>The function returns an error string when <code>b == 0</code>. Then write a caller that handles both the value and the error.</p>
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