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numbers - std::string

Started by thedustbustr Oct 13, 2004 at 7:27 PM 13 replies 1.8k views

Original Post

Oct 13, 2004 at 7:27 PM

I want to write a wrapper so that I can do this:


class A{float b1, b2;}; A a; a.b1=1f;a.b2=2f;
std::string s; s << a;

or something similarly easy and readable, so that it is the equivalent of


char buf[100];sprintf(buf, "A=(%0.1f %0.1f)",a.b1,a.b2);

Operators don't matter (I could care less if its << or +). How might I accomplish this? Thanks. Dustin

Knaperstekt

Oct 13, 2004 at 7:38 PM

One way is to overload the << operator.

string& operator << (string& s, const A& a){   stringstream str;   str << "A=(" << a.b1 << " " << a.b2 << ")";   return s = str.str();}

or something like that

/Boris

Knaperstekt

Oct 13, 2004 at 7:41 PM

Ooops sorry, you didn't want the operator thing... maybee I should read the whole post before replying :-)
sorry 'bout that one.

thedustbustr

Oct 13, 2004 at 7:47 PM

I was unclear - thats exactly what I needed. Thanks :)

jdhardy

Oct 13, 2004 at 7:48 PM

boost::format is what you're looking for, I believe.

EDIT: Okay, I see what you wanted. Geez, you wait five seconds...

thedustbustr

Oct 13, 2004 at 7:57 PM

Having trouble implementing this.

class vector3{float x,y,z;}; //and a lot morestd::string& operator << (std::string& s, const vector3& v){   std::stringstream str;   str << "(" << v.x << " " << v.y << " " << v.z << ")";   return s = str.str();}std::string f; f << "f="<<forward;std::string r; r << "r="<<right;std::string u; u << "u="<<up;state.textbox1->DrawStr(0,738,f.c_str());state.textbox1->DrawStr(0,748,r.c_str());state.textbox1->DrawStr(0,758,u.c_str());error C2784: 'std::basic_ostream<_Elem,_Traits> &std::operator <<(std::basic_ostream<_Elem,_Traits> &,const std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce template argument for 'std::basic_ostream<_Elem,_Traits> &' from 'std::string'

and 99 more errors. fix plz?

Oluseyi

Oct 13, 2004 at 8:33 PM

operator<< is not defined for insertion into std::strings. Here's what you need:

std::ostream operator << (std::ostream & os, const A & a){  // create stringstream, insert/format, etc here  // insert str() value from stringstream into ostream  return os;}

EvilCrap

Oct 13, 2004 at 8:38 PM

notice that you have defined a single operator.
notice that you are trying to stream a string into another string... s << "a" ... is there an operator for that?

joanusdmentia

Oct 13, 2004 at 8:41 PM

Why not just use std::stringstream instead of the std::string? You can also include an operator<< that works with a basic_ostream so it can be used for formatted output to any stream (stringstream, fstream, std::cout, etc).

template<typename Elem, typename Traits>std::basic_ostream<Elem,Traits>& operator<<(std::basic_ostream<Elem,Traits>& out, const vector3& in){    return out << "(" << in.x << " " << in.y << " " << in.z << ")";}std::stringstream f; f << "f="<<forward;std::stringstream r; r << "r="<<right;std::stringstream u; u << "u="<<up;state.textbox1->DrawStr(0,738,f.str().c_str());state.textbox1->DrawStr(0,748,r.str().c_str());state.textbox1->DrawStr(0,758,u.str().c_str());

EDIT: Elaborated with basic_ostream stuff

[Edited by - joanusdmentia on October 13, 2004 9:41:48 PM]

EvilCrap

Oct 13, 2004 at 8:49 PM

howabout adding a member function to the vector class, which will return a string containing data.

thedustbustr

Oct 13, 2004 at 8:55 PM

oluseyi:

std::ostream& operator << (std::ostream& os, const vector3& v){   std::stringstream str;   str << "(" << v.x << " " << v.y << " " << v.z << ")";   os << (s+str.str());   return os;}error C2079: 'str' uses undefined class 'std::basic_stringstream<_Elem,_Traits,_Alloc>'        with        [            _Elem=char,            _Traits=std::char_traits<char>,            _Alloc=std::allocator<char>        ]

I had this error with the original non-ostream attempt as well.

joanusdmentia:

    std::stringstream f; f << "f="<<forward;    state.textbox1->DrawStr(0,738,f.str().c_str());error C2079: 'f' uses undefined class 'std::basic_stringstream<_Elem,_Traits,_Alloc>'        with        [            _Elem=char,            _Traits=std::char_traits<char>,            _Alloc=std::allocator<char>        ]error C2297: '<<' : illegal, right operand has type 'const char [3]'

and also because 'forward' is a vector and needs me to teach it how to format itself.

evilcrap: I did not write the matrix library and I would prefer to leave it untouched

/me is becomg very frustrated

joanusdmentia

Oct 13, 2004 at 9:08 PM

I elaborated above including an operator<< operator.
Did you #include ? It looks like std::basic_stringstream is being forward declared somewhere but not defined.

thedustbustr

Oct 13, 2004 at 9:16 PM

Thanks - I appreciate your patience with me. One final question: is there a way to limit to (and force) 4 decimal places (%0.4f)?

Thanks all for walking me through this
Dustin

Oluseyi

Oct 13, 2004 at 9:26 PM

std::ios_base::width and std::ios::fill.

Knaperstekt

Oct 14, 2004 at 3:51 AM

Quote:
Original post by thedustbustr
Thanks - I appreciate your patience with me. One final question: is there a way to limit to (and force) 4 decimal places (%0.4f)?

Thanks all for walking me through this
Dustin

It's probably easiest to use the setprecision manipulator:

str << setprecision(4) << a.b1;

this does not force 4 decimals though, it just maximizes the output to 4 decimals. (isn't that what %0.4f in fstream does as well?)

/Boris

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