Passing "empty" types as parameters

Hello, What is the recommended way to pass "empty" types as parameters? By empty I mean that they have no fields (or methods, typically), thus really serving as placeholders (e.g. mpl::void_, null_type or even none_t.) Is it better to accept them as values, const values or const references? E.g. void foo(void_); void foo(void_ const); void foo(void_ const&); By "better" I mean in terms of both semantics (making it clear to readers that it's a placeholder) and optimization (since I'm not sure which one is usually easier for a smart compiler to eliminate entirely.) Thanks

Undefined behavior on integer literals overflow ?

Hello I have been reading through the C++ standard and I am surprised to see that users ought to make sure that integer literals never overflow, before compiling their program, as an integer literal overflow results in undefined behavior ! On the other hand, the size of a long int is supposed to be just possibly larger than an int, which in turn is meant to be the native size supported by the underlaying hardware. In other words, you do not really know the size of a long it, but if some literal overflows it, BOOM ! Boy this looks so primitive to me ! And to defy reason even more, i

const static data member from abstract class crashes my program

Hi All, Thanks in advance for any help / insight on this matter. 1) This piece of code (inside the class ReassignCaseRequestHandler : public CaseRequestHandler implementation file) calls a const static data member (CaseRequestHandler::PARAM_CALLING_TRANSACTION) (see code below). The problem I am having is that this static data member (first param to MessageParameter) is uninitialized (bss section). Platform: CentOS 5 gcc: version 4.1.2 20080704 (Red Hat 4.1.2-48) const MessageParameter* ReassignCaseRequestHandler::requestParameters1[] = { new MessageParameter( CaseRequestH

assignment of std::valarray

Hi all. Im trying to parse the C++0x documentation related to the assignment of std::valarray. This code is tested under g++ 4.4.3, VisualStudio2008 and finally VS2010. Three examples of assigning one valarray to another: 1. Pure assignment std::valarray<int> a(2), b(2); std::cerr << &(a[0]) << std::endl; a = b; std::cerr << &(a[0]) << std::endl; 2. Negation + assignment std::cerr << &(a[0]) << std::endl; a = -b; std::cerr << &(a[0]) << std::endl; 3. Computed assignment: std::cerr << &(a[0]) << std::endl; a -= b; std::cerr << &(a[0]) << std::endl; g

"Dependent" non-typename template arguments

Is something like the following allowed? template <typename T, T V> struct foo {}; foo<bool, true> MSVC accepts it happily, but that's never a reliable indicator. What about: template <typename T> struct foo { template <T V> struct bar {}; }; foo<bool>::bar<true> Thanks. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]

returning a std::auto_ptr, a question of style

Hello, I am using a class that returns a pointer to memory that I have to take charge of. I will assign it to a std::auto_ptr so the deallocation is taken care of. My question is, would the function I am calling be better if it was declared to return a std::auto_ptr, rather than a naked pointer? That way it would be self-documenting and I couldn't accidently forget to assign it to a std::auto_ptr. An example of where I am being passed a pointer to memory that I have to take charge of is a virtual ctor that creates an object read from a message queue. The message is in XML (FWIW).

Should C++0x contain a distinct type for UTF-8?

Hi I have posted this question to c.std.c++ a few weeks back but failed to get any useful response. So I thought I'd try a second time: Should C++0x contain a distinct type for UTF-8? Current draft N3092 specifies: + char16_t* for UTF-16 + char32_t* for UTF-32 + char* for execution narrow-character set + wchar_t* for execution wide-character set + unsigned char*, possibly for raw data buffers etc. a) Wouldn't it make sense to have a char8_t where char8_t arrays would hold UTF-8 character sequences exclusively? b) What is the rationale for not including it? cheers, Mart

Any news?

Hi @all, I just wanted to know if there is any news on this topic. In my opinion, cv-qualified constructors would be a nice feature (which has no real workaround). I was hoping for the C++0x standard, but I am very disappointed that I havn't found this issue browsing through the draft. Have I overlooked something? Is this issue still in discussion? Maybe an insider (standardization process) could comment on my question. Thank you very much! Mekiro -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: D

argument packs manipulations

Hello, I am trying to make a class that acts like the following, but in any dimension: template <typename T> struct Vec3 { T tab[3]; Vec3(T const& t1,T const& t2):tab({t1,t2,1}){} Vec3(T const& t1,T const& t2,T const& t3):tab({t1,t2,t3}){} Vec3(T const& t1,T const& t2,T const& t3,T const& t4) :tab({t1/t4,t2/t4,t3/t4}) {} }; My first try is: template <typename T,int d> struct Vec { T tab[d]; template <class... U> Vec(U... u):tab({u...}){ if(sizeof...(u)==d-1) tab[d-1]=1;

functions vs. static member functions

Hello, I'm trying to find out, if the following two codes are equivalent: // 1 class foo { public: static int val; static void bar() { val = 0; } }; // 2 namespace foo { int val; void bar() { val = 0; } } It would be cool, if someone could help me. Regards, Dobi -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]

Unable to read the bits of a 'float' using char*

Hello everyone, I wrote these two functions for reading the bits of a float. However, I can't figure out why the first method shown below works but not the second one. I was sure the second one would also work but that's not the case and I can't seem to find anything wrong with its implementation. I am rather puzzled with this. Could someone please shed some light on this matter? Thanks in advance, Victor Freire // Works OK std::string getBits(const void* number, size_t sz) { unsigned *byte = (unsigned *)number; std::string bits = ""; for(unsigned b = 0; b <

Sorting by a transformation function

Hi, I would like to sort a vector ordered by the return value of a unary transformation function, op. The elements of the vector do not have a copy constructor. The transformation function is expensive and show be called exactly once for each element of the vector. I have a solution, included below. I want to know if this operation is possible using ether standard functions or using boost, and whether my solution could be improved at all. For bonus points, I'd like to include an optional binary predicate as a fourth parameter that can be passed to std::sort. Cheers, Shaun #in

how do you write proxies for rvalues?

As an exercise to familiarize myself with rvalues I decided to write a proxy class for plus operator. It turned out to be more difficult than I thought. Here is what I got (tell me if I did it wrong): template<class T1, class T2> class PlusProxy { T1 t1; T2 t2; public: PlusProxy(T1&& t1, T2&& t2) : t1(std::forward<T1>(t1)), t2(std::forward<T1>(t2)) {} PlusProxy(PlusProxy&& other) : t1(std::forward<T1>(other.t1)), t2(std::forward<T1>(other.t2)) {} PlusP

Can I have your feedback on this general mix-in pattern?

Hi, I needed to implement cloning and saw that I ended up with copy and paste all over the code. I thought that this should be able to be solved with mix-ins. After research and help from you (Is the Mixin pattern accepted in all camps? 6 July) I thought that I probably would find a good solution to this. I found a nice first approach at Alf P. Steinbach's blog. http://alfps.wordpress.com/2010/06/12/cppx-3-ways-to-mix-in-a-generic-cloning-implementation/ The approach that caught my interest was Way #2: Mix-in via a middleman base class. However, I thought the syntax would b

template function Ambiguity.

I developed class vector and class matrix use mingw gcc. then move to vs 2008 get Problem: template<class TYPE,int DIM=3> class Vector ........... template<class TYPE,int COL=4,int ROW=4> class Matrix .............. when call Vector<double,4>*Matrix<double,4,4> Ambiguity in template<class TYPE,int COL,int ROW> Vector<TYPE,COL> operator*(Vector<TYPE,ROW>&v,Matrix<TYPE,COL,ROW> const&m) and template<class TYPE,int DIM,class SCALETYPE> Vector<TYPE,DIM> operator*(Vector<TYPE,DIM> const&v,SCALETYPE t) vs2008 bug or Correct?(mingw gcc is pass). how can I Solve this and don

Why does a base function hide a function in a derived class?

It is normal that a function in derived class hides one in a base class. But, I could explain to myself how a base function could hide one in a derived class. Here is an example: struct A { int x(); }; struct B : A { }; struct C : A { }; struct D : B, C { using A::x; int x(double); void f(); }; void D::f() { x(1.0); //error in gcc v4.4.0, No match for 'A::x(double)' } int main() { } Isn't there a D::x(double)? I would appreciate if someone could explain the error message. Thanks, Jason -- [ See http://www.gotw.ca/resourc

Output Generated From Templates

Hi all, Is there any tool or compiler that will output the result of C++ template instantiation? I think such a thing could be useful for advanced template code development, and also education. Cheers, P. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]

Problem with printing to a file

Hi, I wrote a program in which the main part is a loop with a big number of iterations (the program takes about 1 day to perform the calculations i want) that imitates a flow of time ("professionally speaking": time evolution). After each iteration of the loop I send some data to files. To make the long story short, u can simply imagine a two columns file with time values (proportional to the the loop iterator) in the 1st column and some number/quantity (say, energy) in the 2nd column. So, at the very beginning of the code I open the file: ofstream EnergyStream; EnergyStream.ope

ADL and C++0X Lambdas

Are there supposed to be some strange interactions with C++0X lambdas and ADL? The following code, works in Intel 11.1 and GCC4.5 but fails in VisualStudio 2010. Is this a Microsoft bug? If so, I posted it on https://connect.microsoft.com/VisualStudio/feedback/details/585643/bug-in-adl-lookup-with-c-0x-lambdas#details if anyone wants to vote for it. Thanks namespace X { struct Y{}; template<typename F> void myfunc(F f, Y y) {} } double identity(double x){return x;} int main() { myfunc(identity, X::Y()); //ADL Works as expected myfunc([](double x){return

Vptr

Hi! I have another naive question. I have been trying to understand the polymorphysim implementation in C++. I believe that most of the compilers implement it using vptr and vtables. My question is when for every object of a class (which has a virtual member) if vptr points to the same vtable, why does every object has a copy of the same vptr? For example say X is a class which has a virtual function and a and b are objects of that class both a and b will have a vptr installed in them and this vptr will point to the same vtable Why not have the vptr as a static data member of t

Is there std function that calculates a bounded value?

Is there a boost/std equivalent to: template< typename rc > rc Bounded( rc const& rValue, rc const& rLowest, rc const& rHighest ) { assert( !( rHighest < rLowest ) ); if( rLowest < rValue ) { if( rValue < rHighest ) { return rValue; } else { return rHighest; } } else { return rLowest; } } For strictly ordered types: if ( a<=b ) then ( Bounded( x, a, b ) == min( max( x, a ), b ) ) Also the deduction might be fixed to be based only on the first argument (but it's not crucial for me). itaj -- [ See http://www.gotw.ca/resource

Set implementation in STL

How are sets generally implemented in STL. I have read they are implemented as BST, are they some kind of balanced BST like Red Black trees? -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]

C/C++ query(nested struct)

In case of a nested struct, does the inner struct can access the private member of outer struct. struct str1 { private: int a; public: struct str2 { public: void func(void); }; }; void str1::str2::func() { str1 obj; obj.a=10; //this is working in GNU compiler, but according to Bruce Eckel this should be an error } int main() { str1::str2 obj2; obj2.func(); } Kindly solve my doubt, this really is reported to be an error in Bruce Eckel volume 1 but when i run this on my

C/C++ query

Initializing an array via a variable is restricted in C++. int a=5; int arr[a];//error I checked it in GNU compilers to my astonishment that this code really works. Also suppose an array is declared to hold 5 data.i.e. int arr[5]; Now, if you try to assign value to array element even greater than 5, the compiler doesnt have a problem. arr[7]=10;//this works Kindly solve the doubt as to what is going wrong. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]

Right Endian conversion

Hi to all, I've written down this function for perfoming endian conversion : template<typename T> T swap_endian(const T& source) { T ret = 0; for (int i = 0; i < sizeof(T); ++i) { *(reinterpret_cast<char*>(&ret) + i) = *(reinterpret_cast<char*>(&source) + sizeof(T) - i - 1); } return ret; } It seems to work the most of the times, but I'm not sure when I use it on signed types (int long etc etc) with negative numbers or with positive numbers like 1520 : bigendian 0x05F0 littleendian 0xF005 Is it correct, or has anyone a better routine ?