Related
Is there a (more or less at least) standard int class for c++?
If not so, is it planned for say C++13 and if not so, is there any special reasons?
OOP design would benefit from it I guess, like for example it would be nice to have an assignment operator in a custom class that returns an int:
int i=myclass;
and not
int i=myclass.getInt();
OK, there are a lot of examples where it could be useful, why doesn't it exist (if it doesn't)?
It is for dead reckoning and other lag-compensating schemes and treating those values as 'normal' variables will be nice, hopefully anyway!.
it would be nice to have an assignment operator in a custom class that returns an int
You can do that with a conversion operator:
class myclass {
int i;
public:
myclass() : i(42) {}
// Allows implicit conversion to "int".
operator int() {return i;}
};
myclass m;
int i = m;
You should usually avoid this, as the extra implicit conversions can introduce ambiguities, or hide category errors that would otherwise be caught by the type system. In C++11, you can prevent implicit conversion by declaring the operator explicit; then the class can be used to initialise the target type, but won't be converted implicitly:
int i(m); // OK, explicit conversion
i = m; // Error, implicit conversion
If you want to allow your class to implicitly convert to int, you can use an implicit conversion operator (operator int()), but generally speaking implicit conversions cause more problems and debugging than they solve in ease of use.
If your class models an int, then the conversion operator solution presented by other answers is fine, I guess. However, what does your myclass model?
What does it mean to get an integer out of it?
That's what you should be thinking about, and then you should come to the conclusion that it's most likely meaningless to get an integer without any information what it represents.
Take std::vector<T>::size() as an example. It returns an integer. Should std::vector<T> be convertible to an integer for that reason? I don't think so. Should the method be called getInt()? Again, I don't think so. What do you expect from a method called getInt()? From the name alone, you learn nothing about what it returns. Also, it's not the only method that returns an integer, there's capacity() too.
Implement operator int () for your class
This can be realized by the cast operator. E.g:
class MyClass {
private:
int someint;
public:
operator const int() {
return this->someint;
}
}
No there isn't any standard int class. For things such as BigDecimal you can look at Is there a C++ equivalent to Java's BigDecimal?
As for int, if you really need it, you can create your own. I have never come across an instance where I needed an Integer class.
No, and there won't be any. What you want to do can be done with conversion operator:
#include <iostream>
struct foo {
int x;
foo(int x) : x(x) {}
operator int() { return x; }
};
int main() {
foo x(42);
int y(x);
std::cout << y;
}
No, and there probably won't be.
int i=myclass;
This is covered by conversion operators:
struct MyClass {
operator int() {
return v;
}
int v;
} myclass = {2};
int i = myclass; // i = 2
Not everything has to be 'object oriented'. C++ offers other options.
There are obvious reasons to have a class for int, because int by itself does not allow for the absence of any value. Take for instance a JSON message. It can contain the definition for an object named “foo”, and an integer named “bar”, for example:
{"foo": {"bar": 0}}
Which has the meaning that “bar" is equal to 0 (zero), but if you omit “bar”, like this:
{"foo": {}}
Now it takes on the meaning that “bar” is non-existent, which is a completely different meaning and cannot be represented by int alone. In the old days, if this situation arose, some programmers would use a separate flag, or use a specific integer value to signify that the value was not supplied, or undefined, or non-existent. But whatever you call it, a better way is to have a class for integer which defines the functionality and makes it reusable and consistent.
Another case would be a database table that has an integer column added some time after it’s creation. Records that were added prior to when the new column was added will return null, meaning no value present, and records added after the column’s creation would return a value. You may need to take a different action for null value vs. 0 (zero).
So here's the beginnings of what a class for int or string might look like. But before we get to the code, let's look at the usage as that is why you would create the class in the first place, to make your life easier in the long run.
int main(int argc, char **argv) {
xString name;
xInt age;
std::cout<< "before assignment:" << std::endl;
std::cout<< "name is " << name << std::endl;
std::cout<< "age is " << age << std::endl;
// some data collection/transfer occurs
age = 32;
name = "john";
// data validation
if (name.isNull()) {
throw std::runtime_error("name was not supplied");
}
if (age.isNull()) {
throw std::runtime_error("age was not supplied");
}
// data output
std::cout<< std::endl;
std::cout<< "after assignment:" << std::endl;
std::cout<< "name is " << name << std::endl;
std::cout<< "age is " << age << std::endl;
return 0;
}
Here is the sample output from the program:
before assignment:
name is null
age is null
after assignment:
name is john
age is 32
Note that when the instance of the xInt class has not been assigned a value, the << operator automatically prints "null" instead of zero, and the same applies to xString for name. What you do here is totally up to you. For instance, you might decide to print nothing instead of printing “null”. Also, for the sake of brevity, I've hard coded the assignments. In the real world, you would be gathering/parsing data from a file or client connection, where that process would either set (or not set) the data values according to what is found in the input data. And of course, this program won't actually ever throw the runtime exceptions, but I put them there to give you a flavor of how you might throw the errors. So, one might say, well, why don't you just throw the exception in your data collection process? Well, the answer to that is, with the eXtended class variables (xInt & xString), we can write a generic, reusable, data gathering process and then just examine the data that is returned in our business logic where we can then throw appropriate errors based on what we find.
Ok, so here's the class code to go with the above main method:
#include <iostream>
#include <string>
class xInt {
private:
int _value=0;
bool _isNull=true;
public:
xInt(){}
xInt(int value) {
_value=value;
_isNull=false;
}
bool isNull(){return _isNull;}
int value() {return _value;}
void unset() {
_value=0;
_isNull=true;
}
friend std::ostream& operator<<(std::ostream& os, const xInt& i) {
if (i._isNull) {
os << "null";
} else {
os << i._value;
}
return os;
}
xInt& operator=(int value) {
_value=value;
_isNull=false;
return *this;
}
operator const int() {
return _value;
}
};
class xString {
private:
std::string _value;
bool _isNull=true;
public:
xString(){}
xString(int value) {
_value=value;
_isNull=false;
}
bool isNull() {return _isNull;}
std::string value() {return _value;}
void unset() {
_value.clear();
_isNull=true;
}
friend std::ostream& operator<<(std::ostream& os, const xString& str) {
if (str._isNull) {
os << "null";
} else {
os << str._value;
}
return os;
}
xString& operator<<(std::ostream& os) {
os << _value;
return *this;
}
xString& operator=(std::string value) {
_value.assign(value);
_isNull=false;
return *this;
}
operator const std::string() {
return _value;
}
};
Some might say, wow, that's pretty ugly compared to just saying int or string, and yes, I agree that it's pretty wordy, but remember, you only write the base class once, and then from then on, your code that you're reading and writing every day would look more like the main method that we first looked at, and that is very concise and to the point, agreed? Next you'll want to learn how to build shared libraries so you can put all these generic classes and functionality into a re-usable .dll or .so so that you're only compiling the business logic, not the entire universe. :)
There's no reason to have one, and so there won't be any.
Your cast operator should realize this
An example
class MyClass {
private:
int someint;
public:
operator const int() {
return this->someint;
}
}
Thanks for giving comments to the following.
Class1 { debug(std::ostream&){} };
int main() {
std::vector<Class1*> list1;
// some work to do
}
Target Platform:
Platform(1): Win 7x64, VS2010
Platform(2): Linux x32, g++ 4.4
Q: What should be the correct way to pass "std::cout" to following statement?
std::for_each(list1.begin(),
list1.end(),
"afunction(&Class1::debug, std::cout)");
I previously used "std::cout" inside the debug() function, but later consider to give flexibility for the output of debug message.
Edit: More information: if functor objects is the way to go, how should I implements the functor to cope with multiple classes (those classes have no relationship except the same "debug" function signature)?
Edit(2): Using "std::for_each", is it possible to destroy all objects in list1 by invoking the corresponding destructor for each class directly? (e.g. for_each(l.begin(), l.end(), "Class::~Class1");
Edit(3): As per "pmr" suggested, I make the statement as
std::for_each(l.begin(),
l.end(),
std::bind2nd(std::mem_fn(&Class1::debug), out) );
It compiles and run correctly on linux platform, but failed on VS2010, the code for Class1::debug is
void Class1::debug(const std::ostream& out)
{
out << "some text" << someVar << "some text" << std::endl;
}
The VS error msg is
error C2678: binary '<<' : no operator found which takes a left-hand operand of type 'const std::ostream' (or there is no acceptable conversion)
Any cue?
[Closed]
I now implemented the overloaded operator << for my classes, and the use of debug print function is closed. Thanks very much for all hints given.
Since you are using g++ 4.4 you can't use lambda expressions which would be the first choice (later versions support them, MSVC does as well).
So you need a functor. A functor is a function object, that is a class (or struct) that implements operator(). Like this:
class Debug
{
public:
Debug(ostream& os) : _os(os)
{ }
void operator()(Class1* instance)
{
// will print the pointer, replace with user code
os << instance << endl;
}
private:
ostream& _os;
};
Use like this:
Debug d(cout);
std::for_each(list1.begin(), list1.end(), d);
use lambda instead of function pointers. This is a feature of C++11x and you need to include a flag for compiler to recognise lambda.
std::for_each(list1.begin(), list1.end(), [&debug, &cout]
{
// implementaion
}
);
As GCC does not support lambdas until 4.5, the clearest solution is out of the question.
The second easiest solution when you want to use a lot of generic algorithms is Boost.Lambda http://www.boost.org/doc/libs/1_49_0/doc/html/lambda.html:
for_each(list1.begin(), list.end(), _1->debug(cout));
And finally, the tedious functor solution:
class Output
{
public:
explicit Output(ostream& ios) : os(&ios)
{
}
void operator()(Class1* obj)
{
obj->debug(*os);
}
private:
ostream* os;
};
for_each(list1.begin(), list1.end(), Output(cout));
Personally I think that without C++11 lambdas or Boost Lambdas, for_each is more pain than it's worth. Might as well do a simple loop:
for (vector<Class1*>::iterator it = list1.begin(); it != end; ++it)
(*it)->debug(cout);
C++03:
#include <vector>
#include <functional>
#include <iostream>
#include <algorithm>
struct Foo {
void debug(std::ostream&) {}
};
int main()
{
std::vector<Foo*> foos;
std::for_each(foos.begin(), foos.end(),
std::bind2nd(std::mem_fun(&Foo::debug), std::cout));
return 0;
}
Please note that the binders have been deprecated and boost::bind or
C++11 should be favored. You should really get a newer compiler.
I would like to be able to do:
foo(stringstream()<<"number = " << 500);
EDIT: single line solution is crucial since this is for logging purposes. These will be all around the code.
inside foo will print the string to screen or something of the sort.
now since stringstream's operator<< returns ostream&, foo's signature must be:
foo(ostream& o);
but how can I convert ostream& to string? (or char*).
Different approaches to achieving this use case are welcome as well.
The obvious solution is to use dynamic_cast in foo. But the given
code still won't work. (Your example will compile, but it won't do what
you think it should.) The expression std::ostringstream() is a
temporary, you can't initialize a non-const reference with a temporary,
and the first argument of std::operator<<( std::ostream&, char const*)
is a non-const reference. (You can call a member function on a
temporary. Like std::ostream::operator<<( void const* ). So the code
will compile, but it won't do what you expect.
You can work around this problem, using something like:
foo( std::ostringstream().flush() << "number = " << 500 );
std::ostream::flush() returns a non-const reference, so there are no
further problems. And on a freshly created stream, it is a no-op.
Still, I think you'll agree that it isn't the most elegant or intuitive
solution.
What I usually do in such cases is create a wrapper class, which
contains it's own std::ostringstream, and provides a templated
member operator<< which forwards to the contained
std::ostringstream. Your function foo would take a const
reference to this—or what I offen do is have the destructor call
foo directly, so that the client code doesn't even have to worry about
it; it does something like:
log() << "number = " << 500;
The function log() returns an instance of the wrapper class (but see
below), and the (final) destructor of this class calls your function
foo.
There is one slight problem with this. The return value may be copied,
and destructed immediately after the copy. Which will wreck havoc with
what I just explained; in fact, since std::ostringstream isn't
copyable, it won't even compile. The solution here is to put all of the
actual logic, including the instance of std::ostringstream and the
destructor logic calling foo in a separate implementation class, have
the public wrapper have a boost::shared_ptr to it, and forward. Or
just reimplement a bit of the shared pointer logic in your class:
class LogWrapper
{
std::ostringstream* collector;
int* useCount;
public:
LogWrapper()
: collector(new std::ostringstream)
, useCount(new int(1))
{
}
~LogWrapper()
{
-- *useCount;
if ( *useCount == 0 ) {
foo( collector->str() );
delete collector;
delete useCount;
}
}
template<typename T>
LogWrapper& operator<<( T const& value )
{
(*collector) << value;
return *this;
}
};
Note that it's easy to extend this to support optional logging; just
provide a constructor for the LogWrapper which sets collector to
NULL, and test for this in the operator<<.
EDITED:
One other thing occurs to me: you'll probably want to check whether the
destructor is being called as a result of an exception, and not call
foo in that case. Logically, I'd hope that the only exception you
might get is std::bad_alloc, but there will always be a user who
writes something like:
log() << a + b;
where the + is a user defined overload which throws.
I would suggest you to use this utility struct:
struct stringbuilder
{
std::stringstream ss;
template<typename T>
stringbuilder & operator << (const T &data)
{
ss << data;
return *this;
}
operator std::string() { return ss.str(); }
};
And use it as:
void f(const std::string & s );
int main()
{
char const *const pc = "hello";
f(stringbuilder() << '{' << pc << '}' );
//this is my most favorite line
std::string s = stringbuilder() << 25 << " is greater than " << 5 ;
}
Demo (with few more example) : http://ideone.com/J995r
More on my blog : Create string on the fly just in one line
You could use a proxy object for this; this is a bit of framework, but if you want to use this notation in a lot of places then it may be worth it:
#include <iostream>
#include <sstream>
static void foo( std::string const &s )
{
std::cout << s << std::endl;
}
struct StreamProxy
{
std::stringstream stream;
operator std::string() { return stream.str(); }
};
template <typename T>
StreamProxy &operator<<( StreamProxy &s, T v )
{
s.stream << v;
return s;
}
static StreamProxy make_stream()
{
return StreamProxy();
}
int main()
{
foo( make_stream() << "number = " << 500 );
}
This program prints
number = 500
The idea is to have a little wrapper class which can be implicitely converted into a std::string. The << operator is simply forwarded to the contained std::stringstream. The make_stream() function is strictly speaking not necessary (you could also say StreamProxy(), but I thought it looks a bit nicer.
A couple of options other than the nice proxy solution just presented by Frerich Raabe:
Define a static string stream variable in the header that defines the logging function and use the comma operator in your invocation of the logging function so that this variable is passed rather than the ostream& returned by the stream insertion operator. You can use a logging macro to hide this ugliness. The problem with this solution is that it is a bit on the ugly side, but this is a commonly used approach to logging.
Don't use C++ I/O. Use a varargs C-style solution instead. Pass a format string as the first argument, with the remaining arguments being targets for that format string. A problem with this solution is that even if your compiler is smart enough to ensure that printf and its cousins are safe, the compiler probably won't know that this new function is a part of the printf family. Nonetheless, this is also a commonly used approach.
If you don't mind using macros functions, you can make the logging function accept const string&, and use the following macro
#define build_string(expr) \
(static_cast<ostringstream*>(&(ostringstream().flush() << expr))->str())
And suppose you foo has signature void foo(const string&), you only need the one-liner
foo(build_string("number = " << 500))
This was inspired by James Kanze's answer about static_cast and stringstream.flush. Without the .flush() the above method fails with unexpected output.
Please note that this method should not leak memory, as temporary values, whether in the pointer form or not, are still allocated on the stack and hence destroyed upon return.
Since you're converting to string anyways, why not
void foo(const std::string& s)
{
std::cout << "foo: " << s << std::endl;
}
...
std::stringstream ss;
ss << "number = " << 500;
foo(ss.str());
This is not possible. As the name ostream implies, it is used for output, for writing to it. You could change the parameter to stringstream&. This class has the method str() which returns a std::string for your use.
EDIT I did not read the issue with operator << returning ostream&. So I guess you cannot simply write your statements within the functions argument list but have to write it before.
You can create a small wrapper around std::ostringstream that will convert back to std::string on use, and have the function take a std::string const &. The first approach to this solution can be found in this answer to a different question.
On top of that, you can add support for manipulators (std::hex) if needed.
If I have a function A(), I am interested in finding a convenient method to create a function B() that has the exact same functionality as A(), differing only in name. The new function would be for a one-time use. The intent is to differentiate between calls to the same function in a somewhat primitive sampling profiler, and the duplicated function would only be used in this context. That is, it would never touch production code and only be used for tinkering.
First guess would be a macro that declares a function named B and creates an inlined call to A() inside of it. The problem here is that I'm not aware of a method in GCC to force an arbitrary function call to inline; it seems all inlining options are for function declarations rather than calls.
There may be some esoteric way to do it with templates, or possibly by tricking the compiler into inlining. I'm not sure it's possible. Any thoughts? Unfortunately the new C++ standard is not available, if it would make a difference.
Using templates
template<int x>
void A()
{
// ..
}
int main()
{
A<0>();
A<1>();
return 0;
}
Update
The compiler can be too smart and create only one body for A<0> and A<1>. At least Visual C++ 2010 does it in Release mode. To prevent it, just use the template parameter inside the function template body in logs or asserts. For example,
#include <iostream>
template<int x>
void A()
{
::std::cout << x << std::endl;
// ..
}
int main()
{
A<0>();
A<1>();
auto v0 = A<0>;
auto v1 = A<1>;
::std::cout << v0 << std::endl;
::std::cout << v1 << std::endl;
::std::cout << (v0 == v1) << std::endl;
return 0;
}
This works using templates:
#include <iostream>
template<typename T>
void foo() {
static int x = 0;
std::cout << &x << std::endl;
}
int main(int argc, char **argv) {
foo<int>();
foo<float>();
return 0;
}
If you execute that, you'll see two different values printed, reflecting the compiler generated code for both calls, even though the template parameter is unused. nm on the object file confirms this.
If this is a one-time debug hack, then why not:
#define A_CONTENT \
... // whatever
void A()
{
A_CONTENT
}
void B()
{
A_CONTENT
}
...
A(); // Call to A
B(); // Call to B
Macros are generally grim, but we're not talking about production code here, so who cares?
Having been down this road myself, the short answer is that even if you get the compiler to emit two identical duplicates of a function, the optimizing linker will notice that they're identical and fold them back together into one implementation. (And if you've turned off optimization in the linker, then your profile isn't valid anwyay).
In the context of a sampling profiler, I've found the easier approach is to make two tiny wrappers for the function instead:
void Func() { .... }
_declspec(noinline)
void A_Func( return Func(); }
void B_Func( return Func(); }
void C_Func( return Func(); }
Then when your profiler samples the callstack, you'll be able to differentiate between the different callsites of this function in a very straightforward way..
You could always define a macro, for example in Chromium we do the following to reuse code:
#define CHROMEG_CALLBACK_1(CLASS, RETURN, METHOD, SENDER, ARG1) \
static RETURN METHOD ## Thunk(SENDER sender, ARG1 one, \
gpointer userdata) { \
return reinterpret_cast<CLASS*>(userdata)->METHOD(sender, one); \
} \
\
virtual RETURN METHOD(SENDER, ARG1);
And we call them like:
CHROMEGTK_CALLBACK_1(PageActionViewGtk, gboolean, OnExposeEvent, GdkEventExpose*);
CHROMEGTK_CALLBACK_1(PageActionViewGtk, gboolean, OnButtonPressed, GdkEventButton*);
You can do something similar to do what you wanted. The above example shows us using two different implementations but with one common code base. For GTK callbacks.
It's a little unclear what you're really trying to do, but a really ugly solution would be to declare the body of A as a macro and then you can "inline" this macro within whatever functions you like.
Also, macros are evil. Never use them unless you really have to.
Why do you care so much about inlining it? If you create a wrapper function, there is a pretty good chance the compiler will inline it anyway. At the very least, you're unlikely to get a function frame constructed.
C++11 also lets you do this:
void A() {
...
}
...
auto B = [] () -> void { A(); };
You can now use B syntactically as though it was a function wrapping A.
I like using sentry classes in c++, but I seem to have a mental affliction that results in repeatedly writing bugs like the following:
{
MySentryClass(arg);
// ... other code
}
Needless to say, this fails because the sentry dies immediately after creation, rather than at the end of the scope, as intended. Is there some way to prevent MySentryClass from being instantiated as a temporary, so that the above code either fails to compile, or at least aborts with an error message at runtime?
I can't think of an automatic way to detect if you make this mistake or not. You could always create a macro that expands to the correct thing and use that to declare the sentry instead if you keep using it wrong.
#define MY_SENTRY_CLASS(_X) MySentryClass _sentry(_X)
and then use
MY_SENTRY_CLASS(arg);
or put a post-it on your monitor to remind you.
The only thing you could do is make the constructors private and force access through a helper function. This is far less similar than the initial construction syntax and less likely to be mistaken. You could also allocate on the heap (still a waste) but it's much easier to spot. However, if you want your class to be constructible, you can't stop people constructing rvalues of that type.
Edit: IF you know that MySentryClass always takes an argument, you could disallow construction AND and only allow operator=(arguments). This would force you to do
MySentryClass x;
x = arg;
You could do some kind of method chain for it.
MySentryClass x;
x.SetArg1(arg).SetArg2(arg2).construct();
No, there is no exit from this problem. To make objects on the stack, you have to have public constructors, and if you have public constructors, you can make the mistake you are reporting.
Not sure you'll like this solution, but the solution may well be grep:
find /path/to/project -type f -name \*.cpp -print0 | xargs grep -0 'MySentryClass('
Another thing you could do is use sed or perl to preprocess your source file, replacing MySentryClass( with \n#error MySentryClass used incorrectly\n, which hopefully will give you a line number that's close to where the error is. How to do this depends on your build system.
I think the #define is the best method.
But just as an option for not using #define:
Main
int main()
{
try
{
S arg1;
// This will not compile
// MySentry x1 = MySentry::CreateSentry(arg1);
S arg3;
MySentry x2(MySentry::CreateSentry(arg3));
S arg2;
// This will not compile
// MySentry(arg2);
S arg4;
// This will generate a runtime exception
// It will never call start() or end()
//MySentry::CreateSentry(arg4);
}
catch(std::exception const& e)
{
std::cout << "Exception : " << e.what() << "\n";
}
}
Edited. Now works better.
#include <stdexcept>
#include <iostream>
class S
{
public:
void start() {std::cout << "Start\n";}
void end() {std::cout << "End\n";}
};
class MySentry
{
struct Init
{
Init(S& s) : arg(s),bad(true) {}
~Init() {if (bad) {throw std::runtime_error("Bad usage of MySentry");}}
S& arg;
mutable bool bad;
};
public:
static Init CreateSentry(S& arg) { return Init(arg);}
explicit MySentry(Init const& arg)
: obj(arg.arg)
, bad(false)
{
arg.bad = false;
std::cout << "Created\n";
obj.start();
}
MySentry(MySentry const& rhs)
: obj(rhs.obj)
, bad(false)
{
std::cout << "Copied (this may not appear)\n";
std::cout << "If the optimizer kicks in then the copy may be elided.\n";
// But if it did not optimize out then
// We have to mark the temporaty as bad
// And not call end() in its destructor.
// Note: Never call start() here as it will always be called in the
// main private constrctor above
rhs.bad = true;
}
~MySentry()
{
if (!bad)
{
// Everything working
obj.end();
}
std::cout << "Destroyed\n";
}
private:
S& obj;
mutable bool bad;
};
What you are trying to do is perfectly legal in C++ and I don't think there is a way to disallow it.