前置声明有哪些危险？

我刚刚接受采访。有人问我什么是“前瞻性声明”。然后有人问我是否存在与前瞻性声明相关的危险。

我无法回答第二个问题。在网上搜索并没有出现任何有趣的结果。

那么，有人知道使用前向声明有什么危险吗？

好吧，除了重复的问题之外......

...标准中至少有一个痛处。

如果你打电话delete在指向不完整类型的指针上，您会得到未定义的行为。实际上，析构函数可能不会被调用。

我们可以看到生活工作空间 http://liveworkspace.org/code/fiKs6%242使用以下命令和示例：

// -std=c++11 -Wall -W -pedantic -O2

#include <iostream>

struct ForwardDeclared;

void throw_away(ForwardDeclared* fd) {
   delete fd;
}

struct ForwardDeclared {
   ~ForwardDeclared() {
      std::cout << "Hello, World!\n";
   }
};

int main() {
   ForwardDeclared* fd = new ForwardDeclared();
   throw_away(fd);
}

诊断：

Compilation finished with warnings:
 source.cpp: In function 'void throw_away(ForwardDeclared*)':
 source.cpp:6:11: warning: possible problem detected in invocation of delete operator: [enabled by default]
 source.cpp:5:6: warning: 'fd' has incomplete type [enabled by default] 
 source.cpp:3:8: warning: forward declaration of 'struct ForwardDeclared' [enabled by default]
 source.cpp:6:11: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined

您不想感谢您的编译器向您发出警告吗;)？