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< title > Item 8:优先考虑nullptr而非0和NULL - Effective Modern C++< / title >
2022-06-30 10:23:03 +08:00
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2023-05-06 14:19:14 +08:00
< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "../Introduction.html" > 简介< / a > < / li > < li class = "chapter-item expanded " > < div > 第一章 类型推导< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item1.html" > Item 1:理解模板类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item2.html" > Item 2:理解auto类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item3.html" > Item 3:理解decltype< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item4.html" > Item 4:学会查看类型推导结果< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第二章 auto< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../2.Auto/item5.html" > Item 5:优先考虑auto而非显式类型声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../2.Auto/item6.html" > Item 6:auto推导若非己愿， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第三章 移步现代C++< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item7.html" > Item 7:区别使用()和{}创建对象< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item8.html" class = "active" > Item 8:优先考虑nullptr而非0和NULL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item9.html" > Item 9:优先考虑别名声明而非typedefs< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item10.html" > Item 10:优先考虑限域枚举而非未限域枚举< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item11.html" > Item 11:优先考虑使用deleted函数而非使用未定义的私有声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item12.html" > Item 12:使用override声明重载函数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item13.html" > Item 13:优先考虑const_iterator而非iterator< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item14.html" > Item 14:如果函数不抛出异常请使用noexcept< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item15.html" > Item 15:尽可能的使用constexpr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item16.html" > Item 16:让const成员函数线程安全< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item17.html" > Item 17:理解特殊成员函数函数的生成< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第四章 智能指针< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item18.html" > Item 18:对于独占资源使用std::unique_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item19.html" > Item 19:对于共享资源使用std::shared_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item20.html" > Item 20:当std::shared_ptr可能悬空时使用std::weak_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item21.html" > Item 21:优先考虑使用std::make_unique和std::make_shared而非new< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item22.html" > Item 22:当使用Pimpl惯用法， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第五章 右值引用，移动语义，完美转发< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item23.html" > Item 23:理解std::move和std::forward< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item24.html" > Item 24:区别通用引用和右值引用< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item25.html" > Item 25:对于右值引用使用std::move， < / a > < / li > < li class = "chapter-item expanded " > < a href = "../5
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< h2 id = "条款八优先考虑nullptr而非0和null" > < a class = "header" href = "#条款八优先考虑nullptr而非0和null" > 条款八：优先考虑< code > nullptr< / code > 而非< code > 0< / code > 和< code > NULL< / code > < / a > < / h2 >
< p > < strong > Item 8: Prefer < code > nullptr< / code > to < code > 0< / code > and < code > NULL< / code > < / strong > < / p >
< p > 你看这样对不对：字面值< code > 0< / code > 是一个< code > int< / code > 不是指针。如果C++发现在当前上下文只能使用指针，它会很不情愿的把< code > 0< / code > 解释为指针， < code > 0< / code > 看做< code > int< / code > 而不是指针。< / p >
< p > 实际上，< code > NULL< / code > 也是这样的。但在< code > NULL< / code > 的实现细节有些不确定因素，因为实现被允许给< code > NULL< / code > 一个除了< code > int< / code > 之外的整型类型（比如< code > long< / code > ）。这不常见，但也算不上问题所在。这里的问题不是< code > NULL< / code > 没有一个确定的类型，而是< code > 0< / code > 和< code > NULL< / code > 都不是指针类型。< / p >
< p > 在C++98中， < code > 0< / code > 或< code > NULL< / code > ，它们绝不会调用指针版本的重载函数：< / p >
< pre > < code class = "language-cpp" > void f(int); //三个f的重载函数
void f(bool);
void f(void*);
f(0); //调用f(int)而不是f(void*)
f(NULL); //可能不会被编译， ，
//绝对不会调用f(void*)
< / code > < / pre >
< p > 而< code > f(NULL)< / code > 的不确定行为是由< code > NULL< / code > 的实现不同造成的。如果< code > NULL< / code > 被定义为< code > 0L< / code > （指的是< code > 0< / code > 为< code > long< / code > 类型），这个调用就具有二义性，因为从< code > long< / code > 到< code > int< / code > 的转换或从< code > long< / code > 到< code > bool< / code > 的转换或< code > 0L< / code > 到< code > void*< / code > 的转换都同样好。有趣的是源代码< strong > 表现出< / strong > 的意思（“我使用空指针< code > NULL< / code > 调用< code > f< / code > ”）和< strong > 实际表达出< / strong > 的意思（“我是用整型数据而不是空指针调用< code > f< / code > ”） （ ： ） ， < code > nullptr< / code > ，可能很多程序员还是会继续使用< code > 0< / code > 或< code > NULL< / code > ，哪怕< code > nullptr< / code > 是更好的选择。< / p >
< p > < code > nullptr< / code > 的优点是它不是整型。老实说它也不是一个指针类型，但是你可以把它认为是< strong > 所有< / strong > 类型的指针。< code > nullptr< / code > 的真正类型是< code > std::nullptr_t< / code > ，在一个完美的循环定义以后，< code > std::nullptr_t< / code > 又被定义为< code > nullptr< / code > 。< code > std::nullptr_t< / code > 可以隐式转换为指向任何内置类型的指针，这也是为什么< code > nullptr< / code > 表现得像所有类型的指针。< / p >
< p > 使用< code > nullptr< / code > 调用< code > f< / code > 将会调用< code > void*< / code > 版本的重载函数，因为< code > nullptr< / code > 不能被视作任何整型：< / p >
< pre > < code class = "language-cpp" > f(nullptr); //调用重载函数f的f(void*)版本
< / code > < / pre >
< p > 使用< code > nullptr< / code > 代替< code > 0< / code > 和< code > NULL< / code > 可以避开了那些令人奇怪的函数重载决议，这不是它的唯一优势。它也可以使代码表意明确，尤其是当涉及到与< code > auto< / code > 声明的变量一起使用时。举个例子，假如你在一个代码库中遇到了这样的代码：< / p >
< pre > < code class = "language-cpp" > auto result = findRecord( /* arguments */ );
if (result == 0) {
…
}
< / code > < / pre >
< p > 如果你不知道< code > findRecord< / code > 返回了什么（或者不能轻易的找出），那么你就不太清楚到底< code > result< / code > 是一个指针类型还是一个整型。毕竟，< code > 0< / code > （用来测试< code > result< / code > 的值的那个）也可以像我们之前讨论的那样被解析。但是换一种假设如果你看到这样的代码：< / p >
< pre > < code class = "language-cpp" > auto result = findRecord( /* arguments */ );
if (result == nullptr) {
…
}
< / code > < / pre >
< p > 这就没有任何歧义：< code > result< / code > 的结果一定是指针类型。< / p >
< p > 当模板出现时< code > nullptr< / code > 就更有用了。假如你有一些函数只能被合适的已锁互斥量调用。每个函数都有一个不同类型的指针：< / p >
< pre > < code class = "language-cpp" > int f1(std::shared_ptr< Widget> spw); //只能被合适的
double f2(std::unique_ptr< Widget> upw); //已锁互斥量
bool f3(Widget* pw); //调用
< / code > < / pre >
< p > 如果这样传递空指针：< / p >
< pre > < code class = "language-cpp" > std::mutex f1m, f2m, f3m; //用于f1， ，
using MuxGuard = //C++11的typedef，
std::lock_guard< std::mutex> ;
…
{
MuxGuard g(f1m); //为f1m上锁
auto result = f1(0); //向f1传递0作为空指针
} //解锁
…
{
MuxGuard g(f2m); //为f2m上锁
auto result = f2(NULL); //向f2传递NULL作为空指针
} //解锁
…
{
MuxGuard g(f3m); //为f3m上锁
auto result = f3(nullptr); //向f3传递nullptr作为空指针
} //解锁
< / code > < / pre >
< p > 令人遗憾前两个调用没有使用< code > nullptr< / code > ，但是代码可以正常运行，这也许对一些东西有用。但是重复的调用代码——为互斥量上锁，调用函数，解锁互斥量——更令人遗憾。它让人很烦。模板就是被设计于减少重复代码，所以让我们模板化这个调用流程：< / p >
< pre > < code class = "language-cpp" > template< typename FuncType,
typename MuxType,
typename PtrType>
auto lockAndCall(FuncType func,
MuxType& mutex,
PtrType ptr) -> decltype(func(ptr))
{
MuxGuard g(mutex);
return func(ptr);
}
< / code > < / pre >
2022-11-25 13:43:31 +08:00
< p > 如果你对函数返回类型（< code > auto ... -> decltype(func(ptr))< / code > ）感到困惑不解，< a href = "../1.DeducingTypes/item3.html" > Item3< / a > 可以帮助你。在C++14中代码的返回类型还可以被简化为< code > decltype(auto)< / code > ： < / p >
2022-06-30 10:23:03 +08:00
< pre > < code class = "language-cpp" > template< typename FuncType,
typename MuxType,
typename PtrType>
decltype(auto) lockAndCall(FuncType func, //C++14
MuxType& mutex,
PtrType ptr)
{
MuxGuard g(mutex);
return func(ptr);
}
< / code > < / pre >
< p > 可以写这样的代码调用< code > lockAndCall< / code > 模板（两个版本都可）：< / p >
< pre > < code class = "language-cpp" > auto result1 = lockAndCall(f1, f1m, 0); //错误！
...
auto result2 = lockAndCall(f2, f2m, NULL); //错误！
...
auto result3 = lockAndCall(f3, f3m, nullptr); //没问题
< / code > < / pre >
2023-01-29 11:39:02 +08:00
< p > 代码虽然可以这样写，但是就像注释中说的，前两个情况不能通过编译。在第一个调用中存在的问题是当< code > 0< / code > 被传递给< code > lockAndCall< / code > 模板，模板类型推导会尝试去推导实参类型，< code > 0< / code > 的类型总是< code > int< / code > ，所以这就是这次调用< code > lockAndCall< / code > 实例化出的< code > ptr< / code > 的类型。不幸的是，这意味着< code > lockAndCall< / code > 中< code > func< / code > 会被< code > int< / code > 类型的实参调用，这与< code > f1< / code > 期待的< code > std::shared_ptr< Widget> < / code > 形参不符。传递< code > 0< / code > 给< code > lockAndCall< / code > 本来想表示空指针，但是实际上得到的一个普通的< code > int< / code > 。把< code > int< / code > 类型看做< code > std::shared_ptr< Widget> < / code > 类型给< code > f1< / code > 自然是一个类型错误。在模板< code > lockAndCall< / code > 中使用< code > 0< / code > 之所以失败是因为在模板中，传给的是< code > int< / code > 但实际上函数期待的是一个< code > std::shared_ptr< Widget> < / code > 。< / p >
2022-06-30 10:23:03 +08:00
< p > 第二个使用< code > NULL< / code > 调用的分析也是一样的。当< code > NULL< / code > 被传递给< code > lockAndCall< / code > ，形参< code > ptr< / code > 被推导为整型（译注：由于依赖于具体实现所以不一定是整数类型，所以用整型泛指< code > int< / code > ， < code > long< / code > 等类型），然后当< code > ptr< / code > ——一个< code > int< / code > 或者类似< code > int< / code > 的类型——传递给< code > f2< / code > 的时候就会出现类型错误，< code > f2< / code > 期待的是< code > std::unique_ptr< Widget> < / code > 。< / p >
2023-04-22 11:34:15 +08:00
< p > 然而，使用< code > nullptr< / code > 是调用没什么问题。当< code > nullptr< / code > 传给< code > lockAndCall< / code > 时，< code > ptr< / code > 被推导为< code > std::nullptr_t< / code > 。当< code > ptr< / code > 被传递给< code > f3< / code > 的时候，隐式转换使< code > std::nullptr_t< / code > 转换为< code > Widget*< / code > ，因为< code > std::nullptr_t< / code > 可以隐式转换为任何指针类型。< / p >
2022-06-30 10:23:03 +08:00
< p > 模板类型推导将< code > 0< / code > 和< code > NULL< / code > 推导为一个错误的类型（即它们的实际类型，而不是作为空指针的隐含意义），这就导致在当你想要一个空指针时，它们的替代品< code > nullptr< / code > 很吸引人。使用< code > nullptr< / code > ，模板不会有什么特殊的转换。另外，使用< code > nullptr< / code > 不会让你受到同重载决议特殊对待< code > 0< / code > 和< code > NULL< / code > 一样的待遇。当你想用一个空指针，使用< code > nullptr< / code > ，不用< code > 0< / code > 或者< code > NULL< / code > 。< / p >
< p > < strong > 记住< / strong > < / p >
< ul >
< li > 优先考虑< code > nullptr< / code > 而非< code > 0< / code > 和< code > NULL< / code > < / li >
< li > 避免重载指针和整型< / li >
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