C ++-为std :: string分配null [英] C++ - Assigning null to a std::string

问题描述

我自己学习C ++.我有以下代码，但它给出了错误.

I am learning C++ on my own. I have the following code but it gives error.

#include <iostream>
#include <string>
using namespace std;


int setvalue(const char * value)
{
    string mValue;
    if(value!=0)
    {
       mValue=value;
    }
    else
    {
       mValue=0;
    }
}

int main ()
{
 const char* value = 0;
 setvalue(value);

 cin.get();
 return 0;
}

所以要创建一个接受char指针的函数，我想将一个指针传递给它.该函数将指针分配给其成员变量.我故意传递了一个空指针.以下是我遇到的错误:

So want to create a function which accepts char pointers and I want to pass a pointer to it. The function assigns the pointer to its member variable. I'm passing a null pointer intentionally. Following is the error I'm getting:

 D:\CPP\TestCP.cpp In function `int setvalue(const char*)': 

 note C:\Dev-Cpp\include\c++\3.4.2\bits\basic_string.h:422 candidates are: std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>] 

 note C:\Dev-Cpp\include\c++\3.4.2\bits\basic_string.h:422                 std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const _CharT*) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>] 

 note C:\Dev-Cpp\include\c++\3.4.2\bits\basic_string.h:422                 std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(_CharT) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>] 

基本上是在抱怨:mValue = 0;

it's basically complaining about line: mValue=0;

为什么要抱怨这条线?我不能为字符串分配空值吗?

Why is it complaining about this line? I can't assign a null to a String?

推荐答案

我不能为字符串分配空值吗?

I can't assign a null to a String?

不. std :: string 不是指针类型；不能将其设置为"null".它不能表示缺少值，这是使用空指针表示的值.

No. std::string is not a pointer type; it cannot be made "null." It cannot represent the absence of a value, which is what a null pointer is used to represent.

可以通过为其分配空字符串( s =" 或 s = std :: string()>)或清除它( s.clear()).

It can be made empty, by assigning an empty string to it (s = "" or s = std::string()) or by clearing it (s.clear()).

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