const TypedeffedIntPointer不等于const int * [英] const TypedeffedIntPointer not equal to const int *

问题描述

我有以下C ++代码：

I have the following C++ code:

typedef int* IntPtr;
const int* cip = new int;
const IntPtr ctip4 = cip;

我使用Visual Studio 2008编译，并得到以下错误：

I compile this with Visual Studio 2008 and get the following error:

错误C2440：'initializing'：无法从'const int *'转换为'const IntPtr'

error C2440: 'initializing' : cannot convert from 'const int *' to 'const IntPtr'

显然，我对typedef的理解不是应该的。

Clearly my understanding of typedefs is not what is should be.

我问的原因是，我在一个指针类型STL映射。我有一个函数返回一个const指针，我想用它来搜索地图（使用map :: find（const key_type&））。

The reason I'm asking, I'm storing a pointer type in a STL map. I have a function that returns a const pointer which I would like to use to search in the map (using map::find(const key_type&). Since

const MyType* 

和

const map<MyType*, somedata>::key_type

$ b b

不兼容，我遇到问题。

is incompatible, I'm having problems.

尊敬的
Dirk

Regards Dirk

推荐答案

当你写 const IntPtr ctip4 时，你要声明一个 const-pointer-to-int code> const int * cip 声明一个 pointer-to-const-int 。这些不一样，因此转换是不可能的。

When you write const IntPtr ctip4, you are declaring a const-pointer-to-int, whereas const int * cip declares a pointer-to-const-int. These are not the same, hence the conversion is impossible.

您需要将cip的声明/初始化更改为

You need to change the declaration/initialization of cip to

int * const cip = new int;

要解决此示例中的此问题，您需要更改映射到 const MyType * （无论它是否有意义取决于您的应用程序，但我认为通过在地图中用作键的指针更改MyType对象不太可能） ，或者返回const_casting参数以查找：

To resolve this issue in your example, you need to either change the key type of the map to const MyType * (whether or not it makes sense depends on your application, but I think that changing a MyType object via a pointer used as key in the map is unlikely), or fall back to const_casting the parameter to find:

#include <map>

int main()
{
    const int * cpi = some_func();

    std::map<const int *, int> const_int_ptr_map;
    const_int_ptr_map.find(cpi); //ok

    std::map<int *, int> int_ptr_map;
    int_ptr_map.find(const_cast<int *>(cpi)); //ok
}

这篇关于const TypedeffedIntPointer不等于const int *的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！