变量评估

Question

What does evaluation of variable mean?

int main(){
   int variable;
   variable;
}

What does "variable" do? Why evaluation of "variable" should produce its own value?

Answer 1

Evaluating basically means "executing the code".

In an expression, variable "loads" an int containing the value of that variable into the CPU.

Evaluating the statement variable; "loads" that value to the CPU, which then does nothing with it. (The compiler is smart enough to detect that nothing happens, and will just ignore this. If you're lucky, it'll also produce a warning letting you know that this does nothing and that's suspicious.)

Evaluating the statement int a = variable; "loads" that value to the CPU, and then assigns it to a new local named a .

Answer 2

To "evaluate an expression" means to compute its result and execute its side effects.

Expression variable has no side effects. (Unlike something like x = 1 or printf("abc") ).

It's result has type int (determining the type is not a part of the evaluation, because it never requires runtime computations), and the result has some arbitrary value (actually reading the value would cause undefined behavior, since the variable is uninitialized). This result also has an address, which is the address of variable (you can apply & to this expression and get that address).

In short, variable; doesn't do anything.

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cppreference

Evaluation of each expression includes:

• value computations : calculation of the value that is returned by the expression. This may involve determination of the identity of the object (glvalue evaluation, eg if the expression returns a reference to some object) or reading the value previously assigned to an object (prvalue evaluation, eg if the expression returns a number, or some other value)
• Initiation of side effects : access (read or write) to an object designated by a volatile glvalue, modification (writing) to an object, calling a library I/O function, or calling a function that does any of those operations.

Answer 3

In C++, as with other languages, expressions are evaluated , typically to return a singular value. This happens by following the operator precedence rules to apply operators in the correct order. ¹

In the simplest case:

int x = 2;
int y = x + x * x;

Where the right-hand-side (relative to the = assignment operator) of the expression requires evaluation in each case. In the first statement it's easy, it's just 2 . In the second the evaluation order rules apply and you get 2 + 4 or 6 .

Evaluation is the process of applying every operator in the correct order which will yield a singular value, the result of that expression.

Think about evaluation as similar to how you might when determining the result of a mathematical formula. Different rules apply, but the principles are the same.

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¹ The order arguments are evaluated may not necessarily be the order in which they're defined.