C++ 类提供 RAII 习惯用法。因此您不必关心异常:
void function()
{
// The memory will be freed automatically on function exit
std::vector<int> vector(1000);
// Do some work
}
但是,如果您(由于某些原因)必须使用某些纯 C API,则必须围绕它创建 C++ 包装器或使用 try/catch block
void function()
{
int *arr = (int*)malloc(1000*sizeof(int));
if (!arr) { throw "cannot malloc"; }
try
{
// Do some work
}
catch (...)
{
free(arr); // Free memory in case of exception
throw; // Rethrow the exception
}
// Free memory in case of success
free(arr);
}
即使您使用具有 RAII 惯用法的 C++ 类,有时您也必须编写具有强大异常安全保证的代码:
void function(std::vector<const char*> &vector)
{
vector.push_back("hello");
try
{
// Do some work
vector.push_back("world");
try
{
// Do other work
}
catch (...)
{
vector.pop_back(); // Undo vector.push_back("world")
throw; // Rethrow the exception
}
}
catch (...)
{
vector.pop_back(); // Undo vector.push_back("hello");
throw; // Rethrow the exception
}
}
但是这些结构非常庞大。
有什么方法可以强制在函数退出时运行一些清理代码吗?与 atexit
类似,但在函数范围内...
有没有办法在发生异常时运行一些回滚代码而不使用嵌套的 try/catch block ?
我想要一些像这样工作的运算符或函数:
void function(std::vector<const char*> &vector)
{
int *arr = malloc(1000*sizeof(int));
onexit { free(arr); }
vector.push_back("hello");
onexception { vector.pop_back(); }
// Do some work
vector.push_back("world");
onexception { vector.pop_back(); }
// Do other work
}
如果可以创建此类函数,是否有任何理由避免使用它们?其他编程语言中是否有这样的结构?
最佳答案
我已经创建了实现此功能的宏。它们生成一个局部变量,该变量使用 C++11 lambda 函数在析构函数中运行清理代码。 std::uncaught_exception
函数用于检查当前是否抛出任何异常。创建变量本身不应引发任何异常,因为使用包含通过引用捕获的所有变量的 lambda 来创建变量(此类 lambda 不会在复制/移动构造函数中引发异常)。
#include <exception>
// An object of the class below will run an arbitrary code in its destructor
template <bool always, typename TCallable>
class OnBlockExit
{
public:
TCallable m_on_exit_handler;
~OnBlockExit()
{
if (always || std::uncaught_exception())
{ m_on_exit_handler(); }
}
};
// It is not possible to instantiate an object of the 'OnBlockExit' class
// without using the function below: https://stackoverflow.com/a/32280985/5447906.
// Creating of an object of the 'OnBlockExit' class shouldn't throw any exception,
// if lambda with all variables captured by reference is used as the parameter.
template <bool always, typename TCallable>
OnBlockExit<always, TCallable> MakeOnBlockExit(TCallable &&on_exit_handler)
{
return { std::forward<TCallable>(on_exit_handler) };
}
// COMBINE is needed for generating an unique variable
// (the name of the variable contains the line number:
// https://stackoverflow.com/a/10379844/544790)
#define COMBINE1(X,Y) X##Y
#define COMBINE(X,Y) COMBINE1(X,Y)
// ON_BLOCK_EXIT generates a variable with the name
// in the format on_block_exit##__LINE__
#define ON_BLOCK_EXIT(always, code) \
auto COMBINE(on_block_exit,__LINE__) = MakeOnBlockExit<always>([&]()code)
// Below are target macros that execute the 'code' on the function exit.
// ON_FINALLY will allways execute the code on the function exit,
// ON_EXCEPTION will execute it only in the case of exception.
#define ON_EXCEPTION(code) ON_BLOCK_EXIT(false, code)
#define ON_FINALLY(code) ON_BLOCK_EXIT(true , code)
以下是如何使用这些宏的示例:
void function(std::vector<const char*> &vector)
{
int *arr1 = (int*)malloc(800*sizeof(int));
if (!arr1) { throw "cannot malloc arr1"; }
ON_FINALLY({ free(arr1); });
int *arr2 = (int*)malloc(900*sizeof(int));
if (!arr2) { throw "cannot malloc arr2"; }
ON_FINALLY({ free(arr2); });
vector.push_back("good");
ON_EXCEPTION({ vector.pop_back(); });
auto file = fopen("file.txt", "rb");
if (!file) { throw "cannot open file.txt"; }
ON_FINALLY({ fclose(file); });
vector.push_back("bye");
ON_EXCEPTION({ vector.pop_back(); });
int *arr3 = (int*)malloc(1000*sizeof(int));
if (!arr3) { throw "cannot malloc arr3"; }
ON_FINALLY({ free(arr3); });
arr1[1] = 1;
arr2[2] = 2;
arr3[3] = 3;
}
所有清理代码都以相反的顺序执行(与函数中 ON_FINALLY
/ON_EXCEPTION
宏出现的顺序相反)。仅当控制权超出相应的 ON_FINALLY
/ON_EXCEPTION
宏时,才会执行清理代码。
检查以下链接以查看演示程序执行的输出:http://coliru.stacked-crooked.com/a/d6defaed0949dcc8
关于c++ - 如何在函数导出上运行清理代码?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/48842770/