A tour of C++ Book Note Day4
Just for share my note when I reading the book. If you want to discuss what interests you, you can write in the comments section. If there is a dispute, you can corret it!
1.5 Scope and Lifetime
Scope/local quote27
- 函数形参也可以视作是局部变量
- 因此，就出现了一个问题 -> 函数中的局部变量作为返回值被返回，因此C++中，局部变量的返回不允许被引用。除非是右值引用
Function argument names are considered local names. ^quote27
Scope/class quote28
- 也就是说，只要在类中定义的成员变量(且不是在类中的函数、lambda甚至enum中定义的)，其作用范围就是在类中全局的
if it is defined in a class, outside any function , lambda , or enum class . Its scope extends from the opening { of its enclosing declaration to the matching }. ^quote28
Scope/global quote29
- 如果一个变量定义在任何函数、lambda甚至结构体之内，那么就称其为全局变量，其作用范围也是全局的 -> 全局的命名空间
A name not declared inside any other construct is called a global name and is said to be in the global namespace. ^quote29
1.6 Constants
Constants/const quote30
- const: 其被主要用于指定接口，以便于可以使用指针或者引用来传值而不必担心数据会被修改 -> 尤其是引用
const: This is used primarily to specify interfaces so that data can be passed to functions using pointers and references without fear of it being modified. ^quote30
void function(int* a, int& b) {
// if we not use const to limit pointer and reference, the data may be modified
*a = 5;
b = 6;
}
void function(const int* a, const int& b) {
// if we use const, when we may modify it careless, const will talk to the compiler, we don't want it modified
*a = 6; // wrong
b = 7; // wrong
}
void Contants() {
// if we use const, we can do pass data without fear of it being modified
int a = 2, b = 3;
function(&a, b);
}
Constants/constexpr quote31
- constexpr：大致的意思就是“在编译期计算”。其主要用于指定常量，允许数据放置在只读内存中(不可能损坏) -> 也就是说，允许其在内存中就进行计算，为了更高效。
constexpr: meaning roughly “to be evaluated at compile time.” This is used primarily to specify constants, to allow placement of data in read-only memory (where it is unlikely to be corrupted), and for performance. The value of a constexpr must be calculated by the compiler. ^quote31
constexpr long fibonacii(long num) {
return num <= 2 ? 1 : fibonacii(num - 1) + fibonacii(num - 2);
}
long fibonacii(long num, int) {
return num <= 2 ? 1 : fibonacii(num - 1) + fibonacii(num - 2);
}
void Constants(int) {
std::chrono::time_point start = std::chrono::system_clock::now();
std::cout << fibonacii(45) << "\n";
std::chrono::time_point end = std::chrono::system_clock::now();
std::cout << std::chrono::duration_cast<std::chrono::milliseconds> (end - start).count() << " milliseconds -> constexpr\n";
start = std::chrono::system_clock::now();
std::cout << fibonacii(45, 0) << "\n";
end = std::chrono::system_clock::now();
std::cout << std::chrono::duration_cast<std::chrono::milliseconds> (end - start).count() << " milliseconds -> none\n";
}
[!question] why the constexpr not work?
because the constexpr allow function to be called with non-constant
Constants/constexpr_other quote32
- 被constexpr修饰的函数能够被非常量参数调用，但是如果这样调用之后运行的结果则是一个非常量表达式。我们允许一个被contexpr修饰的函数在不需要常量表达式的上下文中使用非常量参数调用 -> 也就解释了，为什么上文中的contexpr失效的原因，因为其递归后的函数的结果就已经是non-constant了
A constexpr function can be used for non-constant arguments, but when that is done the result is not a constant expression. We allow a constexpr function to be called with non-constant-expression arguments in contexts that do not require constant expressions. ^quote32
Constants/consteval quote33
- 当我们只想使用编译期计算时，我们通常使用consteval而不是constexpr
When we want a function to be used only for evaluation at compile time, we declare it consteval rather than constexpr. ^quote33
consteval long fibonacii(long num) {
return num <= 2 ? 1 : fibonacii(num - 1) + fibonacii(num - 2);
}
// when we declarated it by consteval, it'll be wrong. because the 'fibonacii' function is non-constant after iertation
Constants/pure_function quote34
- 声明contexpr或者consteval的函数是C++对纯函数概念的版本
- 他们不会产生副作用，且只允许使用传过去的信息作为参数。特别是，他们不能修改非局部变量，但他们可以循环且使用自己的局部变量
Functions declared constexpr or consteval are C++’s version of the notion of pure functions.
They cannot have side effects and can only use information passed to them as arguments. In particular, they cannot modify non-local variables, but they can have loops and use their own local variables. ^quote34
constexpr double nth(double x, int n) { // assume 0 <= n
double res = 1;
int i = 0;
while (i<n) { // while-loop: do while the condition is true
res *= x;
++i;
}
return res;
}
Constants quote35
- 在一些地方，语法规则需要常量表达式 -> (例如：数组边界、case标签、模板参数和使用constexpr声明的常量)
- 在其他方面，编译期计算对性能提升非常有用
- 同时，哪怕不考虑性能，不可变性(具有不可更改状态的对象)也是一个重要的设计理念
In a few places, constant expressions are required by language rules (e.g., array bounds, case labels, template value arguments, and constants declared using constexpr).
In other cases, compile-time evaluation is important for performance.
Independent of performance issues, the notion of immutability (an object with an unchangeable state) is an important design concern. ^quote35
|