问将字符串转换为指针
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Stack Overflow用户

提问于 2013-03-18 17:47:12

回答 4查看 9.1K关注 0票数 0

我尝试使用以下代码将指针转换为字符串，并将字符串转换回指针(词法转换)。从指针到字符串的转换工作得很好，但反过来就不行了。

为什么会发生这种情况？是否有其他方法可以将字符串转换为指针？

我并不担心由于字符串格式不正确而导致的错误。我尝试转换为指针的字符串总是通过将指针转换为字符串来生成的。

代码如下：

//TB_ConvertToString.cpp

#include<iostream>
#include<cstdio>
#include<sstream>
#include<string>

//Functions to convert data types to and from strings   
template <typename T> std::string ToString ( T Number )
{
    std::stringstream ss;
    ss << Number;
    return ss.str();
};

template <typename T> 
T FromString ( const std::string &Text )
{
    std::stringstream ss(Text);
    T result;
    ss >> result;
    return result;
}
//-------------------------------------------------


using namespace std;

int main()
{
    int a =10;  
    int* p1 = &a;
    // this works------------
    string s1 = ToString<int *>(p1);
    printf("\n ptr: %s",s1.c_str());
    //----------------------

    //this gives compilation errors -----
    //int * p2 = FromString<int *>(s1);
    //printf("\n ptr: %p",p2);
    //----------------------


    cout<<"\n\n";
    return 0;
};

取消对problem部分的注释会产生以下编译错误：

nehak@Marvin:~/Desktop$ g++ TB_ConvertToString.cpp 
TB_ConvertToString.cpp: In function ‘T FromString(const string&) [with T = int*, std::string = std::basic_string<char>]’:
TB_ConvertToString.cpp:39:33:   instantiated from here
TB_ConvertToString.cpp:21:2: error: no match for ‘operator>>’ in ‘ss >> result’
TB_ConvertToString.cpp:21:2: note: candidates are:
/usr/include/c++/4.6/istream:122:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__istream_type& (*)(std::basic_istream<_CharT, _Traits>::__istream_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:122:7: note:   no known conversion for argument 1 from ‘int*’ to ‘std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&) {aka std::basic_istream<char>& (*)(std::basic_istream<char>&)}’
/usr/include/c++/4.6/istream:126:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__ios_type& (*)(std::basic_istream<_CharT, _Traits>::__ios_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>, std::basic_istream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]
/usr/include/c++/4.6/istream:126:7: note:   no known conversion for argument 1 from ‘int*’ to ‘std::basic_istream<char>::__ios_type& (*)(std::basic_istream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}’
/usr/include/c++/4.6/istream:133:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:133:7: note:   no known conversion for argument 1 from ‘int*’ to ‘std::ios_base& (*)(std::ios_base&)’
/usr/include/c++/4.6/istream:169:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(bool&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:169:7: note:   no known conversion for argument 1 from ‘int*’ to ‘bool&’
/usr/include/c++/4.6/istream:173:7: note: std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(short int&) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.6/istream:173:7: note:   no known conversion for argument 1 from ‘int*’ to ‘short int&’
/usr/include/c++/4.6/istream:176:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(short unsigned int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:176:7: note:   no known conversion for argument 1 from ‘int*’ to ‘short unsigned int&’
/usr/include/c++/4.6/istream:180:7: note: std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(int&) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.6/istream:180:7: note:   no known conversion for argument 1 from ‘int*’ to ‘int&’
/usr/include/c++/4.6/istream:183:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(unsigned int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:183:7: note:   no known conversion for argument 1 from ‘int*’ to ‘unsigned int&’
/usr/include/c++/4.6/istream:187:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:187:7: note:   no known conversion for argument 1 from ‘int*’ to ‘long int&’
/usr/include/c++/4.6/istream:191:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long unsigned int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:191:7: note:   no known conversion for argument 1 from ‘int*’ to ‘long unsigned int&’
/usr/include/c++/4.6/istream:196:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:196:7: note:   no known conversion for argument 1 from ‘int*’ to ‘long long int&’
/usr/include/c++/4.6/istream:200:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long unsigned int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:200:7: note:   no known conversion for argument 1 from ‘int*’ to ‘long long unsigned int&’
/usr/include/c++/4.6/istream:205:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(float&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:205:7: note:   no known conversion for argument 1 from ‘int*’ to ‘float&’
/usr/include/c++/4.6/istream:209:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(double&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:209:7: note:   no known conversion for argument 1 from ‘int*’ to ‘double&’
/usr/include/c++/4.6/istream:213:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long double&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:213:7: note:   no known conversion for argument 1 from ‘int*’ to ‘long double&’
/usr/include/c++/4.6/istream:217:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(void*&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:217:7: note:   no known conversion for argument 1 from ‘int*’ to ‘void*&’
/usr/include/c++/4.6/istream:241:7: note: std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]
/usr/include/c++/4.6/istream:241:7: note:   no known conversion for argument 1 from ‘int*’ to ‘std::basic_istream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}’
/usr/include/c++/4.6/bits/basic_string.tcc:998:5: note: template<class _CharT, class _Traits, class _Alloc> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.6/bits/istream.tcc:957:5: note: template<class _CharT2, class _Traits2> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, _CharT2*)
/usr/include/c++/4.6/bits/istream.tcc:925:5: note: template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, _CharT&)
/usr/include/c++/4.6/istream:709:5: note: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, unsigned char&)
/usr/include/c++/4.6/istream:714:5: note: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, signed char&)
/usr/include/c++/4.6/istream:756:5: note: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, unsigned char*)
/usr/include/c++/4.6/istream:761:5: note: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, signed char*)
nehak@Marvin:~/Desktop$

c++

string

pointers

hex

memory-address

回答 4

Stack Overflow用户

回答已采纳

发布于 2013-03-18 18:08:26

问题是，任何类型的指针都可以隐式转换为void*，但void*不能转换为任何类型的指针。在这种情况下，您应该为所有指针指定FromString函数。像这样

template <typename T> 
T FromString ( const std::string &Text,
    std::enable_if< ! std::is_pointer< T >::value >::type * = nullptr )
{
    std::stringstream ss(Text);
    T result;
    ss >> result;
    return result;
}

template <typename T> 
T FromString ( const std::string &Text,
    std::enable_if< std::is_pointer< T >::value >::type * = nullptr )
{
    std::stringstream ss(Text);
    void * result;
    ss >> result;
    return (T)result;
}

票数 2

Stack Overflow用户

发布于 2013-03-18 17:59:01

您的问题实际上可以缩小到下面这个简单的测试用例：

std::stringstream ss("0xbf845748");
int* p2;
ss >> p2;

这是不起作用的，因为没有>>操作符的重载来初始化指针。要让指针的提取工作，你可以使用void*，或者你甚至可以提取整数，它足够容纳这个地址，然后使用reinterpret_cast<T>让你的指针指向这个地址，只是要非常小心不要以char*过载结束，因为这可能会导致你现在正在处理的更严重的问题。

只需注意，尝试使用存储在std::string中的地址来初始化指针是非常糟糕的主意。在决定让它工作之前，请确保它确实是您在这里想要的。很可能有更合理的方法来避免这种情况。

票数 4

Stack Overflow用户

发布于 2013-03-18 18:17:46

@LihO，谢谢！这是可行的。我不得不在提取指针时使用std::hex。

    string s1 = "0x7fff3e8aee1c";
    stringstream ss;
    ss<<s1;
    long long unsigned int i;
    ss>>std::hex>>i;
    int * i_ptr=reinterpret_cast<int *>(i);

票数 2

页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持

原文链接：

https://stackoverflow.com/questions/15473729

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