How the ternary operator behaves in a C compiler:
intmain()
{
int a=10;
int b=20;
//The ternary operator is an expression, the expression cannot be an lvalue
(a<b?a:b)=30;
printf("a=%d,b=%d",a,b);
return 0;
}
Running the program we found that the compilation failed!
Looking at the reason we see that expressions cannot be lvalues .
How to understand this sentence?
In the C language, the return value of an expression is a number, and the operation result of the expression is placed in the register of the CPU, not in the memory, not a memory address (check the notes for the lvalue and rvalue of C and C++ later) .
That is to say, in the C language, a=10, b=20, and executing the a<b?a:b statement is [10<20? Yes! good! Then return the value of a! ] returns the number "10", then 10=30, assign 30 to 10? What does this mean, it went wrong!
In C++, the expression returns the variable itself, which is the returned variable a.
Compile the same code in C++:
#include<iostream>
using namespace std;
intmain()
{
int a=10;
int b=20;
//The ternary operator is an expression, the expression cannot be an lvalue
(a<b?a:b)=30;
printf("a=%d,b=%d",a,b);
system("pause");
return 0;
}
operation result:
a=30，b=20
Please press any key to continue...
So how to achieve the same effect in C language?
Let's think about it, if we want the expression to be an lvalue, that means the ternary expression should return a memory space ! That is, the first address of the memory! What is the first address of memory? Just a pointer.
After the expression a<b ? a:b is finished, I don't want it to return the value of a, 10. If I can return the address of a, and then assign a value to this address, that's fine.
I can write this: (a < b ? &a : &b) = 30, through this address, the memory space pointed to by the pointer can be indirectly modified, and the value of a can be indirectly modified. Note that an asterisk "* is added before the parentheses. " is to modify the corresponding value. After we modify it and run it, we find that it is the same as C++ running.
intmain()
{
int a=10;
int b=20;
//The ternary operator is an expression, the expression cannot be an lvalue
*(a<b?&a:&b)=30;
printf("a=%d,b=%d",a,b);
return 0;
}
operation result:
a=30，b=20
Press any key to continue
Therefore, the C++ compiler is to help our programmers complete the work of taking the address.
Conclusion :
1. The C language returns the value of the variable, and the C++ language returns the variable itself;
The ternary operator in C language returns the variable value and cannot be used as an lvalue;
The ternary operator in C++ returns the variable itself directly, so it can appear anywhere in the program.
2. Note: If one of the values returned by the ternary operator is a constant value, it cannot be used as an lvalue, such as
（a < b ? 1 : b）= 30；
3. How does C language support features similar to C++?
When the condition of the lvalue: there is memory space, the C++ compiler helps the programmer to take an address.
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