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cppreference Order of evaluation

Guide

1、是基于side effect、value computation而定义的，从最终结果来进行定义，而对执行的过程，没有加以限制，这给予了compiler充分的optimization空间，如果限制地太死，显然，就无法充分发挥compiler、CPU的optimization能力

2、通过这段描述可以看出，C++对Order of evaluation并没有进行严格的限制，它的限制是非常松散的，那C++的限制有哪些呢？C++是基于Sequenced-before relation而建立的，参见下面的"Sequenced-before rules (since C++11)"章节

NOTE: 主要描述C++11，对C++11之前，并不考虑

3、正是这种松散，可能带来一些问题，后面会进行总结

Order of evaluation of any part of any expression, including order of evaluation of function arguments is unspecified (with some exceptions listed below). The compiler can evaluate operands and other subexpressions in any order, and may choose another order when the same expression is evaluated again.

NOTE:

1、为什么C++标准没有对此进行规定？参见"Guide"章节。

No left-to-right or right-to-left evaluation

NOTE:

1、下面这段的描述，是和直觉相悖的。

There is no concept of left-to-right or right-to-left evaluation in C++. This is not to be confused with left-to-right and right-to-left associativity of operators: the expression a() + b() + c() is parsed as (a() + b()) + c() due to left-to-right associativity of operator+, but the function call to c may be evaluated first, last, or between a() or b() at run time:

#include <cstdio>
int a()
{
    return std::puts("a");
}
int b()
{
    return std::puts("b");
}
int c()
{
    return std::puts("c");
}
void z(int, int, int)
{
}
int main()
{
    z(a(), b(), c());       // all 6 permutations of output are allowed
    return a() + b() + c(); // all 6 permutations of output are allowed
}
// gcc test.cpp

NOTE: 下面是在我的计算机上运行的结果

c
b
a
a
b
c

显然是和原网页中的输出不同的

Sequenced-before rules (since C++11)

NOTE:

1、wikipedia C++11 中关于Sequenced-before的描述:

The term sequence point was removed, being replaced by specifying that either one operation is sequenced before another, or that two operations are unsequenced.[25]

通过上述描述可以看出，C++11对于Order of evaluation的限制还是比较松散的: 存在"unsequenced"的情况

2、典型的relation-structure-computation，参见 工程discrete的"Relation"章节

Evaluation of Expressions

NOTE: 在What-is-evaluation-of-expressions章节对这个topic进行了讨论。

Ordering

"sequenced-before" is an asymmetric(非对称), transitive(传递), pair-wise relationship between evaluations within the same thread.

NOTE:

1、"sequenced-before"描述的是"within the same thread"，那inter-thread的ordering呢？有inter-thread sequenced before

1、If A is sequenced before B, then evaluation of A will be complete before evaluation of B begins.

2、If A is not sequenced before B and B is sequenced before A, then evaluation of B will be complete before evaluation of A begins.

NOTE: asymmetric(非对称)

3、If A is not sequenced before B and B is not sequenced before A, then two possibilities exist:

  • evaluations of A and B are unsequenced: they may be performed in any order and may overlap (within a single thread of execution, the compiler may interleave the CPU instructions that comprise A and B)

  • evaluations of A and B are indeterminately(不确定的) sequenced: they may be performed in any order but may not overlap: either A will be complete before B, or B will be complete before A. The order may be the opposite the next time the same expression is evaluated.

    NOTE: 关于 "indeterminately sequenced"，参见下面的 "Rules # 11) "

Rules

NOTE:

1、下面虽然罗列了非常多的rule，其实大多数rule是非常符合我们的惯常思维的

2、少部分不符合惯常思维的rule，需要进行额外的思考

Full expression

NOTE: 其实描述的是两个*full expression*之间的order

1)

Operand and the result of the operator

NOTE: 显然operator是更加关注value的

2)

NOTE: 描述的是

3)

NOTE: 其实描述的是function argument 和 function body 之间的关系

When calling a function (whether or not the function is inline, and whether or not explicit function call syntax is used), every value computation and side effect associated with any argument expression, or with the postfix expression designating the called function, is sequenced before execution of every expression or statement in the body of the called function.

Built-in assignment

NOTE: 显然，assignment是更加关注value的

8)

The side effect (modification of the left argument) of the built-in assignment operator and of all built-in compound assignment operators is sequenced after the value computation (but not the side effects) of both left and right arguments, and is sequenced before the value computation of the assignment expression (that is, before returning the reference to the modified object)

NOTE:

1、先计算left argument 和 right argument的value

2、对于assignment operator而言，它需要首先完成它的side effect(将value写入到memory)，然后再完成它的value computation

Built-in comma operator

9)

Function call

NOTE: 关于 function call的order of evaluation，在下面文章中进行了一些描述:

1、GotW #102: Exception-Safe Function Calls (Difficulty: 7/10)

这篇文章非常好

2、Advantages of using std::make_unique over new operator [duplicate] # A

这些文章，主要收录在如下章节中:

1、Exception-Safe-Function-Call

2、Prefer-use-object-generator

11) A function call that is not sequenced before or sequenced after another function call is indeterminately sequenced (the program must behave as if the CPU instructions that constitute different function calls were not interleaved, even if the functions were inlined).

(since C++17) The rule 11 has one exception: function calls made by a standard library algorithm executing under std::execution::par_unseq execution policy are unsequenced and may be arbitrarily interleaved.

12) The call to the allocation function (operator new) is

1、(until C++17) indeterminately sequenced with respect to the evaluation of the constructor arguments in a new-expression；

2、(since C++17) sequenced before the evaluation of the constructor arguments in a new-expression；

NOTE: 这段话要如何理解？参见:

1、stackoverflow Exception safety and make_unique # A

Sequence point rules (until C++11)

NOTE: 未阅读