Sequence Point - Sequence Points in C and C++

Sequence Points in C and C++

In C and C++, sequence points occur in the following places. (In C++, overloaded operators act like functions, and thus operators that have been overloaded introduce sequence points in the same way as function calls.)

1. Between evaluation of the left and right operands of the && (logical AND), || (logical OR) (as part of short-circuit evaluation), and comma operators. For example, in the expression *p++ != 0 && *q++ != 0, all side effects of the sub-expression *p++ != 0 are completed before any attempt to access q.
2. Between the evaluation of the first operand of the ternary "question-mark" operator and the second or third operand. For example, in the expression a = (*p++) ? (*p++) : 0 there is a sequence point after the first *p++, meaning it has already been incremented by the time the second instance is executed.
3. At the end of a full expression. This category includes expression statements (such as the assignment a=b;), return statements, the controlling expressions of if, switch, while, or do-while statements, and all three expressions in a for statement.
4. Before a function is entered in a function call. The order in which the arguments are evaluated is not specified, but this sequence point means that all of their side effects are complete before the function is entered. In the expression f(i++) + g(j++) + h(k++), f is called with a parameter of the original value of i, but i is incremented before entering the body of f. Similarly, j and k are updated before entering g and h respectively. However, it is not specified in which order f, g, h are executed, nor in which order i, j, k are incremented. Variables j and k in the body of f may or may not have been already incremented. Note that a function call f(a,b,c) is not a use of the comma operator and the order of evaluation for a, b, and c is unspecified.
5. At a function return, after the return value is copied into the calling context. (This sequence point is only specified in the C++ standard; it is present only implicitly in C.)
6. At the end of an initializer; for example, after the evaluation of 5 in the declaration int a = 5;.
7. Between each declarator in each declarator sequence; for example, between the two evaluations of a++ in int x = a++, y = a++. Note that this is not an example of the comma operator.