I have this code, which doesn't compile (EDIT: compiled in VC, but not in GCC):  template&lt;int N&gt; struct List {    template &lt;class C&gt;    void clear() { } };  template &lt;int L&gt; class Test { public:     List&lt;L&gt; list; // &lt;--This line     void method()     {        list.clear&lt;int&gt;(); // &lt;-- Compile error here:  error: expected primary-expression before ‘int’      } };   How can I make it work? It seems the compiler (gcc) doesn't understand the context, because if in the marked line I replace L with a number, like List&lt;1> list; then it compiles fine.
211
In templates, where and why do I have to put typename and template on dependent names? What exactly are dependent names anyway?    I have the following code:  template &lt;typename T, typename Tail&gt; // Tail will be a UnionNode too. struct UnionNode : public Tail {     // ...     template&lt;typename U&gt; struct inUnion {         // Q: where to add typename/template here?         typedef Tail::inUnion&lt;U&gt; dummy;      };     template&lt; &gt; struct inUnion&lt;T&gt; {     }; }; template &lt;typename T&gt; // For the last node Tn. struct UnionNode&lt;T, void&gt; {     // ...     template&lt;typename U&gt; struct inUnion {         char fail[ -2 + (sizeof(U)%2) ]; // Cannot be instantiated for any U     };     template&lt; &gt; struct inUnion&lt;T&gt; {     }; };   The problem I have is in the typedef Tail::inUnion&lt;U&gt; dummy line. I'm fairly certain that inUnion is a dependent name, and VC++ is quite right in choking on it. I also know that I should be able to add template somewhere to tell the compiler that inUnion is a template-id. But where exactly? And should it then assume that inUnion is a class template, i.e. inUnion&lt;U&gt; names a type and not a function?
101584
I'm trying to solve  and I have a for loop that looks like this:   for(int i = low; i &lt;= high; ++i)         {                 res = runalg(i);                 if (res &gt; highestres)                 {                         highestres = res;                 }          }   Unfortunately I'm getting this error when I try to compile with GCC:     3np1.c:15: error: 'for' loop initial   declaration used outside C99 mode   I don't know what C99 mode is. Any ideas?
84760
I am surprised that I can't initialize my fields in structs, why is it like that? Like:  struct MyStruct {     private int a = 90; }   but it's a complie time error. I don't know why it's a problem? Please explain this to me.
983
I have some template code that I would prefer to have stored in a CPP file instead of inline in the header.  I know this can be done as long as you know which template types will be used.  For example:  .h file  class foo { public:     template &lt;typename T&gt;     void do(const T&amp; t); };   .cpp file  template &lt;typename T&gt; void foo::do(const T&amp; t) {     // Do something with t }  template void foo::do&lt;int&gt;(const int&amp;); template void foo::do&lt;std::string&gt;(const std::string&amp;);   Note the last two lines - the foo::do template function is only used with ints and std::strings, so those definitions mean the app will link.  My question is - is this a nasty hack or will this work with other compilers/linkers? I am only using this code with VS2008 at the moment but will be wanting to port to other environments.
45776
I'm trying to do something like the following:  enum E;  void Foo(E e);  enum E {A, B, C};   which the compiler rejects.  I've had a quick look on Google and the consensus seems to be "you can't do it", but I can't understand why.  Can anyone explain?  Clarification 2: I'm doing this as I have private methods in a class that take said enum, and I do not want the enum's values exposed - so, for example, I do not want anyone to know that E is defined as  enum E {     FUNCTIONALITY_NORMAL, FUNCTIONALITY_RESTRICTED, FUNCTIONALITY_FOR_PROJECT_X }   as project X is not something I want my users to know about.  So, I wanted to forward declare the enum so I could put the private methods in the header file, declare the enum internally in the cpp, and distribute the built library file and header to people.  As for the compiler - it's GCC.
7798
According to the , 3rd edition:        I wish to understand why this decision has been made. What's wrong with generic exceptions?  (As far as I know, generics are simply compile-time syntactic sugar, and they will be translated to Object anyway in the .class files, so effectively declaring a generic class is as if everything in it was an Object. Please correct me if I'm wrong.)
11288
I was reading Ivor Horton's Beginning Visual C++ 2008 and many of its CLR examples have this definition for main:  int main(array&lt;System::String ^&gt; ^args)   I went back, page by page, to the beginning of the book to find the first such instance with an explanation what it really means, but couldn't find one.  Obviously it means the same as the standard int main(int argc, char *argv[]), but I'd like to know when and why that ^ is really used, and why it even exists (does it do something that pointers * and references &amp; cannot represent)?
53785
So, every time I have written a lambda expression or anonymous method inside a method that I did not get quite right, I am forced to recompile and restart the entire application or unit test framework in order to fix it. This is seriously annoying, and I end up wasting more time than I saved by using these constructs in the first place. It is so bad that I try to stay away from them if I can, even though Linq and lambdas are among my favourite C# features.  I suppose there is a good technical reason for why it is this way, and perhaps someone knows? Furthermore, does anyone know if it will be fixed in VS2010?  Thanks.
81821
In the  library, they are overloading the () operator.  Is this a convention in C++? For callbacks, etc.?  I have seen this in code of a co-worker (who happens to be a big Boost fan). Of all the Boost goodness out there, this has only led to confusion for me.  Any insight as to the reason for this overload?
30635
I'm a C++ newbie, but I wasn't able to find the answer to this (most likely trivial) question online.  I am having some trouble compiling some code where two classes include each other.  To begin, should my #include statements go inside or outside of my macros?  In practice, this hasn't seemed to matter.  However, in this particular case, I am having trouble.  Putting the #include statements outside of the macros causes the compiler to recurse and gives me "#include nested too deeply" errors.  This seems to makes sense to me since neither class has been fully defined before #include has been invoked.  However, strangely, when I try to put them inside, I am unable to declare a type of one of the classes, for it is not recognized.  Here is, in essence, what I'm trying to compile:  A.h  #ifndef A_H_ #define A_H_  #include "B.h"  class A {     private:         B b;      public:         A() : b(*this) {} };  #endif /*A_H_*/   B.h  #ifndef B_H_ #define B_H_  #include "A.h"  class B {     private:             A&amp; a;      public:         B(A&amp; a) : a(a) {}  };  #endif /*B_H_*/   main.cpp  #include "A.h"  int main() {     A a; }   If it makes a difference, I am using g++ 4.3.2.  And just to be clear, in general, where should #include statements go?  I have always seen them go outside of the macros, but the scenario I described clearly seems to break this principle.  Thanks to any helpers in advance!  Please allow me to clarify my intent if I have made any silly mistakes!
93897
If I have a template function, for example like this:  template&lt;typename T&gt; void func(const std::vector&lt;T&gt;&amp; v)   Is there any way I can determine within the function whether T is a pointer, or would I have to use another template function for this, ie:  template&lt;typename T&gt; void func(const std::vector&lt;T*&gt;&amp; v)   Thanks
92470
I'm wondering about instances when it makes sent to use #define and #if statements. I've known about it for a while, but never incorporated it into my way of coding. How exactly does this affect the compilation?     Is #define the only thing that determines if the code is included when compiled? If I have #define DEBUGme as a custom symbol, the only way to exclude it from compile is to remove this #define statement?
71027
The following code fails to compile stating  "A local variable named 'st' cannot be declared in this scope because it would give a different meaning to 'st', which is already used in a 'child' scope to denote something else":           var l = new List&lt;string&gt;();         l.Find(st =&gt; st.EndsWith("12"));         string st = "why this fails?";   I understand why this won't work:          string preParent = "";         {             string preParent = "Should fail cause we change the meaning";         }   When we do the following we get "CS0103: The name 'postParent' does not exist in the current context":          {             string postParent=string.Empty;         }         postParent = "Should this work?";   What I don't get is why is the compiler smart enough to see that postParent is not within scope, but won't let me define a new variable that has the same name as a variable used within a child scope (which is obviously out of scope at this point).   Is the compiler simple enforcing scope by refusing to let me use the variable? If so this makes sense.  ===========  Edited:   I guess what I also find interesting is how you can have the same variable within two child scopes in a single method, so this is valid:          {             string thisWorks= string.Empty;         }         {             string thisWorks= "Should this work?";         }   I'm just a little curious that you can have two variables with the same name as long as they are at the same level (if you look at scope as a tree). This makes sense because you can have local variables in two methods of the same class with the same name.   I'm just surprised that the compiler is able to differentiate and allow this, while it wouldn't allow the postParent variable. And is this a technical limitation or was this a design decision? That's what I'm really trying to get at;-)
76186
Once it is compiled, is there a difference between:  delegate { x = 0; }   and  () =&gt; { x = 0 }   ?
104370
I want to change member of structure under double pointer. Do you know how?  Example code  typedef struct {     int member; } Ttype;  void changeMember(Ttype **foo) {    //I don`t know how to do it    //maybe    *foo-&gt;member = 1; }
11352
Duplicate:     Maybe this is a very silly question but I don't understand why I can't declare two methods that have the same signature when they have different return types.  public class MyClass {     private double d = 0;      public double MyMethod()     {         return d;     }      public string MyMethod()     {         return d.ToString();     } }   I get a compile error that states that the class already defines a member with the same parameter types.  (Obviously the way I'm using this in my code isn't as simple as my example code...but I think it gets the idea across.)  Am I missing something concerning OO design that makes what I'm trying to do an OOP anti-pattern? Surely the compiler should be able to determine which method I'm trying to use as long as I specifically tell it which one I want.  Given MyClass myClass = new MyClass(); I would expect the following code to work:  double d = myClass.MyMethod(); string s = myClass.MyMethod();   I would expect the following code to have problems:  var v = myClass.MyMethod();   But even in the case of var it should result in a compile error.  Can anyone see what I'm doing wrong here? I'm more than happy to be corrected. :-)
1257
I'm currently refactoring/tidying up some old C code used in a C++ project, and regularly see functions such as:  int f(void)   which I would tend to write as:  int f()   Is there any reason not to replace (void) with () throughout the codebase in order to improve consistency, or is there a subtle difference between the two that I am unaware of?  More specifically, if a virtual member function in C++ is described as:  virtual int f(void)   and a derived class includes a member function:  int f()   is this a valid override?  Additionally, am I likely to encounter any linker problems based on almost identical signatures?