C++中nothrow的介绍及使用
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在C中,使用malloc等分配内存的函数时,一定要检查其返回值是否为”空指针”,并以此作为检查内存操作是否成功的依据,这种Test-for-NULL代码形式是一种良好的编程习惯,也是编写可靠程序所必需的。
在C++中new在申请内存失败时默认会抛出一个std::bad_alloc 异常。如果出现这个异常,那就意味着内存耗尽,或者有其它原因导致内存分配失败。所以,按照C++标准,如果想检查new是否成功,则应该通过try catch捕捉异常。但有些编译器不支持try catch。C++标准化委员会不想遗弃这些Test-for-NULL的代码,所以他们提供了operator new的另一种可选形式---- nothrow,用以提供传统的Failure-yields-NULL行为。
当使用new申请一块内存失败时,抛出异常std::bad_alloc是C++标准中规定的标准行为,所以使用try { p = new int[size];} catch(std::bad_alloc) { … } 的处理方式。
struct nothrow_t {// placement new tag type to suppress exceptions
};extern const nothrow_t nothrow; // constant for placement new tag
其中,nothrow_t通常是一个空结构,其唯一目的是提供编译器一个可根据重载规则识别具体调用的类型。用户一般简单地使用"new(std::nothrow) 类型"(nothrow是一个nothrow_t类型的常量)来调用这个placement new操作符。它与标准new的区别是,new在分配内存失败时会抛出异常,而"new(std::nothrow)"在分配内存失败时会返回一个空指针。
new operator分配内存失败后,缺省的行为不是返回NULL,而是抛出异常std::bad_alloc。所以判断返回值是否为NULL没有任何意义。
std::nothrow: This constant value is used as an argument for operator new and operator new[] to indicate that these functions shall not throw an exception on failure, but return a null pointer instead.
By default, when the new operator is used to attempt to allocate memory and the handling function is unable to do so, a bad_alloc exception is thrown. But when nothrow is used as argument for new, it returns a null pointer instead.
This constant (nothrow) is just a value of type nothrow_t, with the only purpose of triggering an overloaded version of the function operator new (or operatornew[]) that takes an argument of this type.
以下是测试代码:
#include "nothrow.hpp"
#include <iostream>
#include <new> // std::nothrow// reference: http://www.cplusplus.com/reference/new/nothrow/
int test_nothrow1()
{std::cout << "Attempting to allocate 1 MiB...";char* p = new (std::nothrow) char[1048576];if (p == 0)std::cout << "Failed!
";else {std::cout << "Succeeded!
";delete[] p;}return 0;
}// reference: http://en.cppreference.com/w/cpp/memory/new/nothrow
int test_nothrow2()
{try {while (true) {new int[100000000ul]; // throwing overload}}catch (const std::bad_alloc& e) {std::cout << e.what() << '
';}while (true) {int* p = new(std::nothrow) int[100000000ul]; // non-throwing overloadif (p == nullptr) {std::cout << "Allocation returned nullptr
";break;}}return 0;
}
GitHub:
https://github.com/fengbingchun/Messy_Test