C++中的运行时和编译时常量

运行时常数：

这些是常数，其各自的值只能在运行源代码时获知或计算。运行时常量比编译时常量慢一点，但比编译时常量灵活。但是，一旦初始化，就不能更改这些常量的值。

下面是用于说明运行时常量的程序：

C++

// C++ program to illustrate
// Run-time Constants
#include 
using namespace std;
  
// Driver Code
int main()
{
    // Input a variable
    double electonmass;
    cin >> electonmass;
  
    // Define a constant
    // and initialize it at
    // run-time
    const double electon_mass{ electonmass };
  
    // Known to the compiler
    // at the run-time
    cout << electon_mass << endl;
  
    return 0;
}

输出：

2.07335e-317

编译时常数：

这些是常数，其各自的值在编译源代码时是已知的或已计算出的值。编译时常量比运行时常量快，但不如运行时常量灵活。

下面是用于说明编译时常数的程序：

C++

// C++ program to illustrate
// compile-time constants
#include 
using namespace std;
  
// Driver Code
int main()
{
    // Declare and initialize
    // compile time constant
    const double electron_q{ 1.6e-19 };
  
    // Value known to compiler
    // at compile-time
    cout << electron_q << endl;
  
    return 0;
}

输出：

1.6e-19

运行时和编译时常量之间的差异

	Compile-time constants	Run-time constants
1.	A compile-time constant is a value that is computed at the compilation-time.	Whereas, A runtime constant is a value that is computed only at the time when the program is running.
2.	A compile-time constant will have the same value each time when the source code is run.	A runtime constant can have a different value each time the source code is run.
3	It is generally used while declaring an array size.	It is not preferred for declaring an array size.
4	If you use const int size = 5 for defining a case expression it would run smoothly and won’t produce any compile-time error.	Here, if you use run-time constant in your source code for defining case expression then it will yield a compile-time error.
5	It does not produces any compile time error when used for initializing an enumerator.	Same compilation error, if runtime constant is used for initializing an enumerator.

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