I'm trying to implement a manipulator for my stream class. I don't know much about manipulators, but I think I'm doing everything right. The relevant parts of the code are below:
class stream
{
public:
stream& operator<<(bool b) { // send bool value and return *this; }
stream& operator<<(const char str[]) { // send string and return *this }
};
inline stream& endl(stream& s)
{
return s << "\r\n";
}
class stream stream;
int main()
{
stream << endl;
}
I don't know what I'm doing wrong, but instead of calling endl the compiler is calling stream::operator<<(bool). Any idea?
Seeing stream << endl; the compiler has to pick an overload from the operator <<s you provided. endl is not convertible to const char *, but it is convertible to bool, so that's what you get.
You probably meant to add an overload
stream& operator<<(stream &(*function)(stream &)) {
return function(*this);
}
inside of class stream that handles function pointers correctly.
Since your endl is neither a bool nor a const char[] (it is a free function), it is implicitly converted to a bool(true) and the following function is called:
stream::stream& operator<<(bool b)
You could define endl to be of a special type endl_t and define the right operator for it:
#include <iostream>
#include <string>
#include <array>
//Just make the operators `explicit`. See [this excellent answer on SO](http://stackoverflow.com/a/8239402/5470596).
class stream
{
public:
stream& operator<<(bool b) { std::cout << "bool\n" ; return *this; }
stream& operator<<(const char str[]) { std::cout << "const char[]\n" ; return *this; }
};
struct endl_t {} endl;
stream& operator<<(stream& s, endl_t)
{
s << "\r\n";
return s;
}
int main()
{
stream s;
s << endl; // prints "const char[]"
}
Live on coliru
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Error ocurred with the following try to operator overloading:
#include<iostream>
#include<string>
#include<ostream>
using namespace std;
class Dollar
{
private:
float currency, mktrate, offrate;
public:
Dollar(float);
float getDollar() const;
float getMarketSoums() const;
float getofficialSoums() const;
void getRates();
// In the following function I was trying to overload "<<" in order to print all the data members:
friend void operator<<(Dollar &dol, ostream &out)
{
out << dol.getDollar() << endl;
out << dol.getMarketSoums() << endl;
out << dol.getofficialSoums() << endl;
}
};
Dollar::Dollar(float d)
{
currency = d;
}
float Dollar::getDollar() const
{
return currency;
}
float Dollar::getMarketSoums() const
{
return mktrate;
}
float Dollar::getofficialSoums() const
{
return offrate;
}
void Dollar::getRates()
{
cin >> mktrate;
cin >> offrate;
}
int main()
{
Dollar dollar(100);
dollar.getRates();
// In this line I am getting the error. Could you please help to modify it correctly?
cout << dollar;
system("pause");
return 0;
}
You have to pass std::ostream object as the first parameter to the insertion operator << not as the second one as long as you are calling it that way:
friend void operator << (ostream &out, Dollar &dol);
You should make the object passed in to the insertion operator constant reference as long as this function is only prints and not intending to modify the object's members:
friend void operator << (ostream &out, const Dollar& dol);
So pass by reference to avoid multiple copies and const to avoid unintentional modification.
If you want to invoke to get it work the way you wanted you can do this:
friend void operator<<(const Dollar &dol, ostream &out){
out << dol.getDollar() << endl;
out << dol.getMarketSoums() << endl;
out << dol.getofficialSoums() << endl;
}
And in main for example:
operator << (dollar, cout); // this is ok
dollar << cout; // or this. also ok.
As you can see I reversed the order of calling the insertion operator to match the signature above. But I don't recommend this, it is just to understand more how it should work.
I'm having troubles understanding the reason why the compiler accuses error, when the return type of a << operator overload is std::string. Could you please help me understand?
Bellow is an reproducible example, which gives a gigantic error.
class XY
{
int X__;
int Y__;
public:
XY(int x, int y):X__(x), Y__(y){}
~XY(){}
std::string operator<<(const XY_cartesiano& c)
{
std::stringstream ss;
ss << "{ " << X__ << ", " << Y__ << " }";
return ss.str();
}
int x() const{return X__;}
int y() const{return Y__;}
};
void main()
{
XY a(1,2);
std::cout << a;
}
Let's take something like this as an example:
cout << "My number is " << 137 << " and I like it a lot." << endl;
This gets parsed as
((((cout << "My number is ") << 137) << " and I like it a lot.") << endl);
In particular, notice that the expression cout << "My number is " has to evaluate to something so that when we then try inserting 137 with << 137 the meaning is "take 137 and send it to cout."
Imagine if cout << "My number is " were to return a string. In that case, the << 137 bit would try to use the << operator between a string on the left-hand side and an int on the right-hand side, which isn't well-defined in C++.
The convention is to have the stream insertion operator operator << return a reference to whatever the left-hand side stream is so that these operations chain well. That way, the thing on the left-hand side of << 137 ends up being cout itself, so the above code ends up essentially being a series of chained calls to insert things into cout. The signature of these functions therefore usually look like this:
ostream& operator<< (ostream& out, const ObjectType& myObject) {
// ... do something to insert myObject into out ... //
return out;
}
Now, everything chains properly. Notice that this function is a free function, not a member function, and that the left-hand side is of type ostream and the right-hand side has the type of your class in it. This is the conventional way to do this, since if you try overloading operator << as a member function, the left-hand side will be an operand of your class type, which is backwards from how stream insertion is supposed to work. If you need to specifically access private fields of your class in the course of implementing this function, make it a friend:
class XY {
public:
...
friend ostream& operator<< (ostream& out, const XY& myXY);
};
ostream& operator<< (ostream& out, const XY &myXY) {
...
return out;
}
Correct way to overload << operator in your case is
ostream& operator<<(ostream& os, const XY& c)
{
os << c.X__ <<" "<< c.Y__ ;
return os;
}
You have overloaded operator<< in a way that's incompatible with the conventions you must follow when you intend to use the operator with a std::ostream object like std::cout.
In fact, your operator<<'s signature has nothing to do with streams at all! It is just a member function of XY which takes another XY (which it then does not use), returns a string and has an unsual name. Here's how you would theoretically call it:
XY a(1,2);
XY b(1,2);
std::string x = (a << b);
The correct way to overload operator<< for use with streams is to make the operator a non-member function, add a stream reference parameter and return a stream reference to the stream argument. You also do not need a string stream; you write directly to the stream you get:
#include <iostream>
class XY
{
int x;
int y;
public:
XY(int x, int y) : x(x), y(y) {}
int X() const { return x; }
int Y() const { return y; }
};
std::ostream& operator<<(std::ostream& os, XY const& c)
{
os << "{ " << c.X() << ", " << c.Y() << " }";
return os;
}
int main()
{
XY a(1,2);
std::cout << a;
}
Given a class
class ostreamWrapper
{
private:
ostream * str;
public:
ostreamWrapper operator << (const char *);
}
where ostream * str will point to std::cout and ostreamWrapper operator << (const char *) sends the given text to the wrapped ostream str.
In this case, I can only instance << "const char * text" and no other printable data. Unlike directly <<ing a std::cout or std::cerr.
How can the operator method be implemented so it accepts any type of data just as std::cout or std::cerr directly do?
First, write a public operator<< template so it can accept any type and simply forward it to the wrapped ostream.
template <class T>
ostreamWrapper& operator<<(T&& x) {
*str << std::forward<T>(x);
return *this;
}
Second, in order to accept insertion of stream manipulator templates such as std::endl, add a second public operator<< that specifically accepts manipulators intended for the wrapped ostream:
ostreamWrapper& operator<<(ostream& (*manip)(ostream&)) {
*str << manip;
return *this;
}
Omitting the second overload will cause insertion of overloaded manipulators or manipulator templates to fail with "ambiguous overload" or similar error messages.
See an example of the proposed implementation, it would deduce the template parameter type and print accordingly, if you could use C++11 see #Brian answer:
#include <iostream>
using namespace std;
class ostreamWrapper {
private:
ostream* str;
public:
ostreamWrapper(ostream* str_v) : str(str_v) {}
template <typename T>
ostreamWrapper& operator<<(const T& t) {
if (str)
*str << t;
return *this;
}
};
int main() {
ostreamWrapper osw(&std::cout);
osw << 1 << " texto " << std::string(" otro texto ") << 1.2345;
return 0;
}
I have a custom class called Stream
class Stream
public:
Stream& operator<<(int i) { stream_ << i; return *this;}
template <typename CustomClass>
Stream& operator<<(const CustomClass& c) { stream_ << c.toString() /* assume this template always have toString(); return *this; }
private:
std::stringstream stream_;
};
This is a very basic example of what I actually have. And I am trying to set std::ios_base flags like following:
Stream() << 1 << std::hex << 2;
using operator;
Stream& operator<<(std::ios_base& b) { stream_.setf(b.flags()); return *this; }
from what I understand, because std::hex returns std::ios_base so it should call this and set streams' flag. But it always call the template. Note: If i remove this template, everything works just as good as you would expect but is there a way to have both?
Please feel free to ask further if you need more clarification
IOStream manipulators are not objects of type std::ios_base, they are functions that take and return std::ios_base references. So when you want to do stream insertion regarding these objects, you have to overload for function pointers:
Stream& operator<<(std::ios_base& (*manip)(std::ios_base&))
// ^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^
{
manip(this->stream);
return *this;
}
I want to write a function that outputs something to a ostream that's passed in, and return the stream, like this:
std::ostream& MyPrint(int val, std::ostream* out) {
*out << val;
return *out;
}
int main(int argc, char** argv){
std::cout << "Value: " << MyPrint(12, &std::cout) << std::endl;
return 0;
}
It would be convenient to print the value like this and embed the function call in the output operator chain, like I did in main().
It doesn't work, however, and prints this:
$ ./a.out
12Value: 0x6013a8
The desired output would be this:
Value: 12
How can I fix this? Do I have to define an operator<< instead?
UPDATE: Clarified what the desired output would be.
UPDATE2: Some people didn't understand why I would print a number like that, using a function instead of printing it directly. This is a simplified example, and in reality the function prints a complex object rather than an int.
You can't fix the function. Nothing in the spec requires a compiler to evaluate a function call in an expression in any particular order with respect to some unrelated operator in the same expression. So without changing the calling code, you can't make MyPrint() evaluate after std::cout << "Value: "
Left-to-right order is mandated for expressions consisting of multiple consecutive << operators, so that will work. The point of operator<< returning the stream is that when operators are chained, the LHS of each one is supplied by the evaluation of the operator to its left.
You can't achieve the same thing with free function calls because they don't have a LHS. MyPrint() returns an object equal to std::cout, and so does std::cout << "Value: ", so you're effectively doing std::cout << std::cout, which is printing that hex value.
Since the desired output is:
Value: 12
the "right" thing to do is indeed to override operator<<. This frequently means you need to either make it a friend, or do this:
class WhateverItIsYouReallyWantToPrint {
public:
void print(ostream &out) const {
// do whatever
}
};
ostream &operator<<(ostream &out, const WhateverItIsYouReallyWantToPrint &obj) {
obj.print(out);
}
If overriding operator<< for your class isn't appropriate, for example because there are multiple formats that you might want to print, and you want to write a different function for each one, then you should either give up on the idea of operator chaining and just call the function, or else write multiple classes that take your object as a constructor parameter, each with different operator overloads.
You want to make MyPrint a class with friend operator<<:
class MyPrint
{
public:
MyPrint(int val) : val_(val) {}
friend std::ostream& operator<<(std::ostream& os, const MyPrint& mp)
{
os << mp.val_;
return os;
}
private:
int val_;
};
int main(int argc, char** argv)
{
std::cout << "Value: " << MyPrint(12) << std::endl;
return 0;
}
This method requires you to insert the MyPrint object into the stream of your choice. If you REALLY need the ability to change which stream is active, you can do this:
class MyPrint
{
public:
MyPrint(int val, std::ostream& os) : val_(val), os_(os) {}
friend std::ostream& operator<<(std::ostream& dummy, const MyPrint& mp)
{
mp.os_ << mp.val_;
return os_;
}
private:
int val_;
std::ostream& os_
};
int main(int argc, char** argv)
{
std::cout << "Value: " << MyPrint(12, std::cout) << std::endl;
return 0;
}
You have two options. The first, using what you already have is:
std::cout << "Value: ";
MyPrint(12, &std::cout);
std::cout << std::endl;
The other, which is more C++-like, is to replace MyPrint() with the appropriate std::ostream& operator<<. There's already one for int, so I'll do one just a tad more complex:
#include <iostream>
struct X {
int y;
};
// I'm not bothering passing X as a reference, because it's a
// small object
std::ostream& operator<<(std::ostream& os, const X x)
{
return os << x.y;
}
int main()
{
X x;
x.y = 5;
std::cout << x << std::endl;
}
There's no way to do what you're expecting there because of the order the functions are evaluated in.
Is there any particular reason you need to write directly to the ostream like that? If not, just have MyPrint return a string. If you want to use a stream inside MyPrint to generate the output, just use a strstream and return the result.
First, there is no reason not to pass in the ostream by reference rather than by a pointer:
std::ostream& MyPrint(int val, std::ostream& out) {
out << val;
return out;
}
If you really don't want to use std::ostream& operator<<(std::ostream& os, TYPE), you can do this:
int main(int argc, char** argv){
std::cout << "Value: ";
MyPrint(12, std::cout) << std::endl;
return 0;
}
After changing the pointer to a reference, you can do this:
#include <iostream>
std::ostream& MyPrint(int val, std::ostream& out) {
out << val;
return out;
}
int main(int, char**) {
MyPrint(11, std::cout << "Value: ") << std::endl;
return 0;
}
The syntax for MyPrint is essentially that of an unrolled operator<< but with an extra argument.
In your case the answer is obviously:
std::cout << "Value: " << 12 << std::endl;
If that isn't good enough, please explain what output you want to see.