C++核心准则ES.42: 使用指针时要简单且直接-技术圈

ES.42: Keep use of pointers simple and straightforward

ES.42: 使用指针时要简单且直接

Reason（原因）

Complicated pointer manipulation is a major source of errors.

复杂的指针操作是错误的主要来源之一。

Note（注意）

Use gsl::span instead. Pointers should only refer to single objects. Pointer arithmetic is fragile and easy to get wrong, the source of many, many bad bugs and security violations.span is a bounds-checked, safe type for accessing arrays of data. Access into an array with known bounds using a constant as a subscript can be validated by the compiler.

使用gls::span。指针只应该用于参照单独的对象。指针运算脆弱且易错，会导致特别特别多的错误和安全违反。span类型提供了具有边界检查的、安全的访问数组数据的手段。使用常数下标访问一个已知边界的数组的操作可以在编译时检查。

Example, bad（反面示例）

void f(int* p, int count)
{
    if (count < 2) return;

    int* q = p + 1;    // BAD

    ptrdiff_t d;
    int n;
    d = (p - &n);      // OK
    d = (q - p);       // OK

    int n = *p++;      // BAD

    if (count < 6) return;

    p[4] = 1;          // BAD

    p[count - 1] = 2;  // BAD

    use(&p[0], 3);     // BAD
}

Example, good（范例）

void f(span a) // BETTER: use span in the function declaration
{
    if (a.size() < 2) return;

    int n = a[0];      // OK

    span q = a.subspan(1); // OK

    if (a.size() < 6) return;

    a[4] = 1;          // OK

    a[a.size() - 1] = 2;  // OK

    use(a.data(), 3);  // OK
}

Note（注意）

Subscripting with a variable is difficult for both tools and humans to validate as safe.span is a run-time bounds-checked, safe type for accessing arrays of data.at() is another alternative that ensures single accesses are bounds-checked. If iterators are needed to access an array, use the iterators from a span constructed over the array.

使用变量下标的情况下确保安全性无论对工具还是人都很困难。span是访问数组数据的安全类型，可以提供执行时的范围检查。at()是确保单独访问时进行边界检查的另一种方式。如果迭代器需要访问数组，使用来自构建在数组之上的span的迭代器。

Example, bad（反面示例）

void f(array a, int pos)
{
    a[pos / 2] = 1; // BAD
    a[pos - 1] = 2; // BAD
    a[-1] = 3;    // BAD (but easily caught by tools) -- no replacement, just don't do this
    a[10] = 4;    // BAD (but easily caught by tools) -- no replacement, just don't do this
}

Example, good（范例）

Use a span:

使用span：

void f1(span a, int pos) // A1: Change parameter type to use span
{
    a[pos / 2] = 1; // OK
    a[pos - 1] = 2; // OK
}

void f2(array arr, int pos) // A2: Add local span and use that
{
    span a = {arr.data(), pos};
    a[pos / 2] = 1; // OK
    a[pos - 1] = 2; // OK
}

Use at():

使用at()：

void f3(array a, int pos) // ALTERNATIVE B: Use at() for access
{
    at(a, pos / 2) = 1; // OK
    at(a, pos - 1) = 2; // OK
}

Example, bad（反面示例）

void f()
{
    int arr[COUNT];
    for (int i = 0; i < COUNT; ++i)
        arr[i] = i; // BAD, cannot use non-constant indexer
}

Example, good（范例）

Use a span:

使用span：

void f1()
{
    int arr[COUNT];
    span av = arr;
    for (int i = 0; i < COUNT; ++i)
        av[i] = i;
}

Use a span and range-for:

使用span和范围for：

void f1a()
{
     int arr[COUNT];
     span av = arr;
     int i = 0;
     for (auto& e : av)
         e = i++;
}

Use at() for access:

使用at()访问：

void f2()
{
    int arr[COUNT];
    for (int i = 0; i < COUNT; ++i)
        at(arr, i) = i;
}

Use a range-for:

使用范围for：

void f3()
{
    int arr[COUNT];
    int i = 0;
    for (auto& e : arr)
         e = i++;
}

Note（注意）

Tooling can offer rewrites of array accesses that involve dynamic index expressions to use at() instead:

工具可以建议重写包含动态索引运算的数组访问代码，转而使用at()。

static int a[10];

void f(int i, int j)
{
    a[i + j] = 12;      // BAD, could be rewritten as ...
    at(a, i + j) = 12;  // OK -- bounds-checked
}

Example（示例）

Turning an array into a pointer (as the language does essentially always) removes opportunities for checking, so avoid it。

将数组转换为指针（像语言一直在做的那样）放弃了检查的机会，应该避免。

void g(int* p);

void f()
{
    int a[5];
    g(a);        // BAD: are we trying to pass an array?
    g(&a[0]);    // OK: passing one object
}

If you want to pass an array, say so:

如果想传递一个数组，这样做：

void g(int* p, size_t length);  // old (dangerous) code

void g1(span av); // BETTER: get g() changed.

void f2()
{
    int a[5];
    span av = a;

    g(av.data(), av.size());   // OK, if you have no choice
    g1(a);                     // OK -- no decay here, instead use implicit span ctor
}

Enforcement（实施建议）

Flag any arithmetic operation on an expression of pointer type that results in a value of pointer type.
标记对指针表达式进行数学运算然后得到指针类型的结果的情况。
Flag any indexing expression on an expression or variable of array type (either static array or std::array) where the indexer is not a compile-time constant expression with a value between 0 and the upper bound of the array.
如果一个索引不是编译时可确定其值区间为0到数组上限的常量表达式，对数组类型变量或表达式的索引表达式的风险进行提示。
Flag any expression that would rely on implicit conversion of an array type to a pointer type.
提示表达式依靠从数组到指针的隐式类型转换，提示。

This rule is part of the bounds-safety profile.

本规则是边界安全规则群组的一部分。

原文链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es42-keep-use-of-pointers-simple-and-straightforward

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