将字符串转换为指针

Question

我试图将指针转换为字符串，并将字符串转换为使用以下代码的指针（词法转换）。从指针到字符串的转换工作正常，但不是相反。

这是为什么发生？ 有没有其他方法可以将字符串转换为指针？

我并不担心由不正确的字符串格式造成的错误。我试图转换为指针的字符串将始终通过将指针转换为字符串来生成。

下面的代码：

//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; 
}; 

取消注释的问题部分提供了以下编译错误：

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$ 

Answer 1

的问题是，任何类型的指针可以是隐式转换为void*，但void*可以不被转换为任何类型的指针。在你的情况下，你应该专注于所有指针的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; 
} 

Answer 2

你的问题可能实际上缩小到这个简单的测试案例：

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

哪个不行，因为没有这样的超载>>运算符将初始化您的指针。为了完成指针工作的提取，你可以使用void*，或者你甚至可以提取整数，这个数字足够大以保存这个地址，然后使用reinterpret_cast<T>来使你的指针指向这个地址，只是非常小心，不要结束与char*超载，因为这可能会导致你现在正在处理的更严重的问题。

请注意，试图通过使用存储在std::string中的地址来初始化指针实在是个坏主意。在你决定使它工作之前，确保它真的是你想要的。如何避免这种情况最有可能更合理。

Answer 3

@LihO，谢谢！这样可行。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);