الدرس 23 · Modern C++

const and constexpr

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A C++ program lives twice: once while the compiler builds it, and again every time it runs. constexpr asks the compiler to work a value out while it builds, so it is baked into the program and can size an array. const only promises that a value never changes after it is set, and that value may come from run time. Follow a ten-line program as sq(4) is computed by the compiler, argc arrives when the program runs, and two small mistakes stop the build.

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01In constexpr int N = sq(4);, when is sq(4) worked out?

A constexpr variable must be initialized by a constant expression, so the compiler evaluates sq(4) during the build and bakes 16 into the program.

02Which line compiles?

argc is only known when the program runs. const accepts a run-time value and just forbids later changes. constexpr, array sizes and static_assert all need a value known at compile time.

03const int k = argc * 3; and later k = 5;. What happens?

Writing to a const object is caught by the compiler (GCC: assignment of read-only variable 'k'), so no program is made.

04sq is declared constexpr. What happens in return sq(k); when k comes from argc?

constexpr on a function means it may be evaluated at compile time. With a run-time argument it simply runs like any other function. A consteval function (C++20) would refuse.

05Which statement about const and constexpr is true?

A constexpr variable is implicitly const. The difference is when the value is known: constexpr requires compile time; const allows run time.

المصطلحات الأساسية

المسرد

Compile time: While the compiler is building your program from source code. Nothing has run yet.
Run time: While the built program is running on the CPU. Values such as argc or user input only exist now.
const: A promise that an object is read-only after it is initialized. Its value may be worked out at run time.
constexpr variable: A variable whose value the compiler must work out while building. It is also const.
constexpr function: A function the compiler may run at compile time when its arguments are constant; otherwise it runs normally at run time.
Constant expression: An expression the compiler can evaluate while building, needed for array sizes, static_assert and template arguments.
static_assert: A check the compiler makes at compile time. If its condition is false, the build stops.
consteval: C++20: marks a function that must run at compile time; calling it with a run-time value is an error.

تفاصيل إضافية

الملاحظات والمصادر

Compile time and run time: the compiler (g++ or clang++) turns app.cpp into the program file app; running ./app happens later, as often as you like. Values a program reads when it runs, such as argc or user input, cannot be known while it is built.
The standard guarantees that N's value is known to the compiler, so it can be used where a constant expression is required. Whether N also gets a slot in memory is up to the compiler; in practice 16 is written into the machine code wherever N is used.
A const int initialized from a constant, such as const int L = 16;, is also usable in constant expressions. constexpr makes the requirement explicit and checked: if the value cannot be computed at compile time, the build fails.
GCC and Clang accept int a[n] with a run-time n as an extension (a variable-length array, borrowed from C); standard C++ does not allow it, and -Wvla or -pedantic warns about it.
Exact messages differ: for the write to k, GCC says "assignment of read-only variable 'k'" and Clang says "cannot assign to variable 'k' with const-qualified type 'const int'". For sq(argc), Clang says "constexpr variable 'M' must be initialized by a constant expression"; GCC also rejects it, in its own words.
The program on the card is real, standard C++20. It compiles with no warnings under g++ or clang++ -std=c++20 -Wall -Wextra; ./app exits with status 9 and ./app a b with 81.

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