Converting strings to pointers

Question

I'm trying to convert a pointer to string, and the string back to the pointer (lexical casting) using the following code. The conversion from pointer to string works fine, but not the other way round.

Why does this happen? Is there some other way I can convert a string to a pointer?

I'm not worried about errors caused by incorrect format of the string. The string I'm trying to convert to a pointer will always be generated by converting a pointer to string.

Here's the code:

//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;
};

Un-commenting the problem section gives the following compilation errors:

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

I ended up using std::stoul.

void *p = std::stoul(str.c_str(), 0, 16);

Answer 2

Your problem could be actually narrowed down to this simple test case:

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

which doesn't work, because there is no such overload of >> operator that would initialize your pointer. To make this extraction of pointer work, you could use void* or you could even extract the integral number, that is big enough to hold this address and then use reinterpret_cast<T> to make your pointer point to this address, just be really careful not to end up with char* overload because that might cause you much worse problems that you are dealing with now.

Just note that trying to initialize a pointer by using the address stored in std::string is really bad idea. Before you decide to make it work, make sure it is really what you want here. There are most likely much more reasonable ways how to avoid this.

Answer 3

@LihO, thanks! That works. I had to use std::hex while extracting the pointer though.

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

Answer 4

The problem is that any type of pointer can be implicit cast to void*, but void* can not be cast to any type of pointer. In your case you should specialize FromString function for all pointers. Like this

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;
}