I am trying to find what g++ compiler parameters to use for compiling this sample c++ code:
#include <stdio.h>
#include <functional>
struct mystruct
{
int a;
int operator()(int y) { return y + 1; }
};
int main()
{
std::function<mystruct> foo;
return 0;
}
In cpp reference is written that std::function template parameter can be "...or other function objects". I work on large project containing several std::function and project is buildable with g++. I am trying to build it under VS2015 but compiler on such code complains:
error C2027: use of undefined type 'std::_Get_function_impl<_Fty>'
with
[
_Fty=mystruct
]
when I try to compile above small sample under g++ with -std=c++11 it also comlains:
error: aggregate 'std::function a' has incomplete type and cannot be defined
So I think in our large buildable project g++ probably has switched some extension which provides such capability.
Probably you mixed up the template argument with a function object to be stored. I believe what did you want to write is
std::function<int(int)> foo{mystruct{}};
Answer is that functor cannot be template parameter to other functor. Thanks for all posts.
Related
I want to set the array length to be the minimum of a constant and a generic like this:
template <int foo> struct Bar{
void my_func( int const (&my_array)[std::min(5, foo)] ) { /*...*/ }
};
This code compiles with clang++ but not g++ and I need my code to work with both. The error g++ gives is: error: array bound is not an integer constant before ']' token. How I can set the length of this array to be the minimum of foo and 5?
When I use clang++ I run into the problem that I can't get anything to bind to my_array. I want to run something like:
int main() {
static const int var[5] = {0,1,2,3,4};
Bar<5> bar;
bar.my_func(var);
}
But when I try to compile this code in clang++ I get: error: reference to type 'const int [*]' could not bind to an lvalue of type 'const int [5]'.
If I get rid of the std::min() stuff and replace it with foo the code compiles and runs fine.
Notes:
To get this code to compile you'll need to #include <algorithm> or similar to access std::min.
I don't think that this being part of a template should matter but when I try similar things with non-template function such as:
const int const_five = 5;
void new_func( int const (&my_array)[std::min(5,const_five)] ) { /*...*/ }
g++ says: error: variable or field 'new_func' declared void and clang++ says candidate function not viable: no known conversion from 'const int [5]' to 'const int [std::min(5, const_five)]' for 1st argument which both look like similar problems.
For int const (&my_array)[std::min(5, foo)] to compile, you need a version of std::min which is constexpr. It is since C++14.
Check the default value for -std of gcc and clang you use (its version-dependant). Ultimately, compile with -std=c++14.
Provided by StoryTeller, a nice working MCVE.
Keep it simple:
[foo < 5 ? foo : 5]
I have the following code
#include <boost/function.hpp>
#include <boost/bind.hpp>
class Foo {
public:
int getIfoo();
};
int Foo::getIfoo() {
return 5;
}
int main () {
boost::function<int (Foo)> getIntFoo;
getIntFoo = boost::bind( &Foo::getIfoo, _1 );
return 0;
}
When I compile with the following command g++ TestBoostBind.cpp I've got the following error
/includes/boost_1_60_0/boost/bind/mem_fn_template.hpp:35:36: error: invalid conversion from ‘const Foo*’ to ‘Foo*’ [-fpermissive]
BOOST_MEM_FN_RETURN (u.*f_)();
~~~~~~~^~
I'm confused about the source of the error whether it's originally from my code or the boost library. Does anyone know what the error means and how to fix it? I use g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0 and boost.1.60
When binding to a member function, the first argument needs to be a pointer or a reference to the object to call the function on. It specifically can't be a value (an actual object instance). The boost::bind function have special cases for these two alternatives to generate the correct objects. It does not have any special case for passing by value.
Therefore you need to define getIntFoo as a function taking a pointer to Foo:
boost::function<int (Foo*)> getIntFoo;
Or a reference:
boost::function<int (Foo&)> getIntFoo;
You could try to use std::mem_fn to achieve the same goal:
Foo f;
std::function<int(Foo &)> getIntFoo = std::mem_fn(&Foo::getIfoo);
int ret = getIntFoo(f);
or if you need pointer argument, std::function could resolve this for you:
Foo f;
std::function<int(Foo *)> getIntFoo = std::mem_fn(&Foo::getIfoo);
int ret = getIntFoo(&f);
boost have its own alternative
I'm writing a library code on top of Boost Geometry library. My class should be templated on the coordinate type (usually int/float/double etc.).
The code below (stripped down to bare minimum) doesn't compile and I get a compilation error that doesn't help me.
The code:
#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point.hpp>
template <typename CoordType>
class MyClass {
public:
typedef boost::geometry::model::point<CoordType, 2, boost::geometry::cs::cartesian> MyPoint;
CoordType getX(const MyClass<CoordType>::MyPoint &p) const { return p.get<0>(); }
};
The error:
test.cpp: In member function 'CoordType MyClass<CoordType>::getX(const MyClass<CoordType>::MyPoint&) const':
test.cpp:8:82: error: expected primary-expression before ')' token
I'm compiling this code with: g++ -I./boost_1_54_0 test.cpp -o test.o. I used different versions of G++ 4.5.2/4.7.2/4.8.1, but I still get the same error.
What am I missing here?
Thanks in advance.
Using the free function boost::geometry::get<0>(p); recommended in the boost docs circumvents this problem.
I agree with the answer of us2012, using boost::geometry::get<0>() is recommended.
The actual problem was that the template keyword was missing, so this:
{ return p.template get<0>(); }
would have fixed the problem.
I have a problem compiling a c++ program with gcc version 4.6.3; I can compile this program with microsoft compiler (v 9.0) without problems.
This program is using some of my libraries I always used with microsoft compiler.
problem is when I try to pass a reference as argument that is a subtype of another: pseudo example here:
class ObjManager{..}
class SubObjMng : public ObjManager{
public:
inline SubObjMng() : ObjManager(0, ... ){}
};
class Test{
public:
Test(int i, ObjManager &obj_mng);
}
int main(){
SubObjMng myobjmng;
Test t(0, myobjmng); //GCC ERROR HERE
}
output of the error is (real output for my program):
globals.h:227:40: error: no matching function for call to cdk::HashMap::HashMap(unsigned int, cdk::PtrObjMng, cdk::cstrObjMng)
globals.h:227:40: note: candidates are:
contrib/cdklib/cdk_struct.h:485:12: note: cdk::HashMap::HashMap(uint32_t, cdk::ObjManager&, cdk::ObjManager&)
contrib/cdklib/cdk_struct.h:485:12: note: no known conversion for argument 2 from cdk::PtrObjMng to cdk::ObjManager&
anyone can help?
thanks!
cdk::PtrObjMng should inherit from cdk::ObjMng, making polymorphism possible through references. Otherwise this is a no-go according to what the compiler says.
Of course this might not be the root of your problem, I wish we could see the implementation of your constructor.
FC++ is a library for functional programming in C++. I am using it on MinGW with gcc 4.5.0. When I use the more basic features, I have no problems. But one of the more advanced features is giving me a template-related error (or maybe there's just a problem with my code...couldn't be!).
Does anybody have FC++ working with 4.5.0? See anything wrong with my code?
The link text is solidly out of date, though it shows a history of issues with gcc and template specialization. The link text isn't more up to date.
Here's my code:
#include <iostream>
#include "prelude.h"
using namespace fcpp;
using namespace std;
struct TwoTimes {
template <class T>
struct Sig : public FunType<T,T> {};
template <class F>
F operator() (const F& x) const { return 2*x; };
} twoTimes;
int main(int argc, char* argv[] )
{
cout << compose(twoTimes,twoTimes)(3) << endl;
return 0;
}
The error I get is:
In file included from full.h:14:0,
from lambda.h:38,
from operator.h:29,
from function.h:23,
from reuse.h:14,
from list.h:31,
from prelude.h:32,
from y.cxx:2:
smart.h: In instantiation of 'fcpp::FunctoidTraits<TwoTimes>':
prelude.h:142:74: instantiated from 'fcpp::impl::XCompose::Sig<TwoTimes, TwoTimes>'
full.h:94:53: instantiated from 'fcpp::Full2<fcpp::impl::XCompose>::Sig<TwoTimes, TwoTimes>'
y.cxx:18:46: instantiated from here
smart.h:103:7: error: no type named 'Type' in 'struct
fcpp::impl::NeededASmartFunctoidButInsteadGot<TwoTimes, false>'
In file included from y.cxx:2:0:
prelude.h: In instantiation of 'fcpp::impl::XCompose::Sig<TwoTimes, TwoTimes>':
full.h:94:53: instantiated from 'fcpp::Full2<fcpp::impl::XCompose>::Sig<TwoTimes, TwoTimes>'
y.cxx:18:46: instantiated from here
prelude.h:142:74: error: 'fcpp::FunctoidTraits<TwoTimes>::max_args' is not a valid
template argument for type 'int' because it is a non-constant expression
In file included from lambda.h:38:0,
from operator.h:29,
from function.h:23,
from reuse.h:14,
from list.h:31,
from prelude.h:32,
from y.cxx:2:
full.h: In instantiation of 'fcpp::Full2<fcpp::impl::XCompose>::Sig<TwoTimes, TwoTimes>':
y.cxx:18:46:
instantiated from here
full.h:94:53: error: no type named 'Arg1Type' in 'struct
fcpp::impl::XCompose::Sig<TwoTimes, TwoTimes>'
y.cxx: In function 'int main(int, char**)':
y.cxx:18:46: error: no match for call to '(fcpp::Compose) (TwoTimes&, TwoTimes&)'
Well .... the links you refer to are seven years old. In dog and compiler years, that is a good lifetime.
If you want functional programming with (current) C++ compilers, maybe some of the Boost libraries may be of interest?
If the answer is still needed to this, the struct TwoTimes needs to be wrapped to work with FC++. I have worked a lot with FC++ over several years. The best thing I can do is point to here on the C2 wiki where I have discussed this:
http://c2.com/cgi/wiki?FunctoidsInCpp
Any posting there I will see.