将增量运算符应用于常量变量时​​，为什么会出现编译器错误

Question

If I declare a constant variable

int const n=100; cout<<n+1<<endl;

The console shows the value as 101

but when I write a code like this:

int const n=100;
n++;
cout<<n<<endl;

There is a compile time error :

 main.cpp: In function 'int main()': main.cpp:6:5: error: increment of read-only variable 'n' 

Is the second case different from the first case?

Answer 1

Is the second case different from the first case?

Yes they are fundamentally different.

int const n=100; 
n++;

The increment operator obviously cannot applied for a const (ant) variable, because the const keyword prevents it to be changed after the initial definition. That's why the compiler error is issued.

In the other case, the variable itself isn't changed, but another temporary value is created when it's passed to the operator<<() of std::cout .

Answer 2

In the first case the compiler is asked to compute the output of adding a constant to an integer. This causes no error.

In the second case, the compiler is asked to change the value of a constant. This is illegal and results in a compiler error.