C++核心准则ES.42: 使用指针时要简单且直接
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2020-05-09 23:21
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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