C+为什么避免定义没有明确语义的概念

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T.20:避免定义没有明确语义的"概念"

Reason（原因）

Concepts are meant to express semantic notions, such as "a number", "a range" of elements, and "totally ordered." Simple constraints, such as "has a + operator" and "has a > operator" cannot be meaningfully specified in isolation and should be used only as building blocks for meaningful concepts, rather than in user code.

概念意在表现某种有语义的观念，例如"数字"，元素的"范围"，和"完全有序"等。简单的约束，例如有+运算符，有>运算符不能算作被独立，明确地定义，只应用于某个明确概念的组成部分，而不是在代码中直接使用。

Example, bad (using TS concepts)

反面示例（使用TS概念）

template
concept Addable = has_plus;    // bad; insufficient

template auto algo(const N& a, const N& b) // use two numbers
{
    // ...
    return a + b;
}

int x = 7;
int y = 9;
auto z = algo(x, y);   // z = 16

string xx = "7";
string yy = "9";
auto zz = algo(xx, yy);   // zz = "79"

Maybe the concatenation was expected. More likely, it was an accident. Defining minus equivalently would give dramatically different sets of accepted types. This Addable violates the mathematical rule that addition is supposed to be commutative: a+b == b+a.

也许期待的操作就是连结。更有可能的是，这只是一个意外。对等地定义减操作将会提供一套明显不同的可接受类型。这个可加性违反了加法运算满足交换律（a+b==b+a）这个规则。

Note（注意）

The ability to specify a meaningful semantics is a defining characteristic of a true concept, as opposed to a syntactic constraint.

定义明确语义的能力是真正的概念所具备的明确特征，而不是句法约束。

Example (using TS concepts)

示例（使用TS概念）

template
// The operators +, -, *, and / for a number are assumed to follow the usual mathematical rules
concept Number = has_plus
                 && has_minus
                 && has_multiply
                 && has_divide;

template auto algo(const N& a, const N& b)
{
    // ...
    return a + b;
}

int x = 7;
int y = 9;
auto z = algo(x, y);   // z = 16

string xx = "7";
string yy = "9";
auto zz = algo(xx, yy);   // error: string is not a Number

Note(注意)

Concepts with multiple operations have far lower chance of accidentally matching a type than a single-operation concept.

相比只有一个操作的概念，包含多个操作的概念偶然和某个类型匹配的可能性会大为降低。

Enforcement（实施建议）

• Flag single-operation concepts when used outside the definition of other concepts.

• 标记处于其他概念定义范围之外的单操作概念。

• Flag uses of enable_if that appears to simulate single-operation concepts.

• 标记将enable_if用于模拟单操作概念的情况。
  

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