Static assertion static_ assert

1. Assertion

Assertion （assertion） Is a common means of programming . In general , Assertion is to put a judgment expression whose return value always needs to be true in the statement , Used to exclude situations that should not occur logically in the design .

such as ： A function always needs to input parameters within a certain range , Then the programmer can use assertions on this parameter , To force the program to exit when an exception occurs to this parameter , So as to avoid the program falling into logical confusion .

In some sense , Assertions are not necessary for normal programs , But for program debugging , Assertions often help program developers quickly locate program errors that violate certain prerequisites .

If we want to C++ Using assertions in programs , You need to include header files in the program or <assert.h>, The header file gives us assert macro , Used to assert at run time .

• eg：

#include <iostream>
#include <cassert>
using namespace std;

//  Create a... Of a specified size  char  Type array 
char* createArray(int size)
{
    
/* Assertion used  assert(expression) ,  This is a macro , Its argument is an expression , This expression usually returns a Boolean value ,  And the expression must be  true  The program can continue to run down , Otherwise, it will be interrupted directly .*/
    //  Determine whether the array size is greater than by asserting 0
    assert(size > 0);	//  Must be greater than 0,  Otherwise, the program will be interrupted 
    char* array = new char[size];
    return array;
}

int main()
{
    
    char* buf = createArray(0);
    //  What is used here is vs Security functions provided ,  You can also use  strcpy
    strcpy_s(buf, 16, "hello, world!");
    cout << "buf = " << buf << endl;
    delete[]buf;
    return 0;
}

• test ： If createArray The parameter is less than or equal to 0, Program run to 16 Line exit directly , You will see the prompt message as shown in the following figure ：
  

2. Static assertions

In the above example, we used assertions assert. but assert Is a runtime assertion , That is, it only works when the program is running .

Static assertions static_assert, Statics are assertions that can be checked at compile time , There is no need to reference the header file when using . Another benefit of static assertions is , You can customize the error message when the assertion is violated .

• Static assertions are very simple to use , It takes two parameters ：

Parameters 1： Assertion expression , This expression usually needs to return a bool value
Parameters 2： Warning message , It is usually a string , In violation of the assertion （ Expression for false） Prompt this message when

Because based on VS The calculated byte size is different from the theoretical value , The following program is based on 64 position Linux To test , Use static assertions to verify that the current operating system is 32 position ：

• eg：

// assert.cpp
#include <iostream> 
using namespace std;

int main()
{
    
    //  The font reason looks like a =,  Actually, these are two =
    static_assert(sizeof(long) == 4, " error ,  No 32 Bit platform ...");
    cout << "64bit Linux  Pointer size : " << sizeof(char*) << endl;
    cout << "64bit Linux long  size : " << sizeof(long) << endl;

    return 0;
}

• test ：
  

The static assertion condition is judged as true, The program can continue to execute downward .

• Because the expression of static assertion is detected at the compilation stage , Therefore, the variable cannot appear in its expression , That is, the expression must be a constant expression .

3. 32 Bit system and 64 Comparison of data types of bit system ：

• Reference resources ： Static assertions static_assert