Re: How to Define a global const class variable in MFC?

Daniel James <>
Mon, 01 Sep 2008 11:47:46 +0100
In article news:<>, Electronic75 wrote:

I generally do not use global variable much but in this case I like
to define a series of constant commands for program. the program
talks to different embedded devices and user at the beginning selects
a device that is connected to via USB. Now for talking I need a big
set of commands for each device these commands not only include their
command codes but many parameters, priorities, conversion algorithm

What everyone else has been telling you is true ... if a thing is
constant you cannot assign to it at runtime.

The only way that you can give a value to a constant in C++ (and it's the
same in C, by the way) is to initialize it at the point where it is declared.

So, you can write:

  const int i = 42;
but you can't write

  const int i;
  i = 42;
because the second line is an assignment to a constant, which is not allowed.

It may be clearer to think what this would mean, if it were permitted:

  const int i = 17;
  i = 42;
Here you can see that the constant i is set to 17 in the declaration, and
that the program later tries to change it to 42 -- and this is obviously
nonsense. The thing to understand is that this is really no different from
the earlier case ... whether or not you give i a value at its declaration
you can never give it a value later in the program.

Another point is that you can't assign anything to anything at file scope,
you can only make assignments inside a function. That is, if your source
file looks like this:

--- file.cpp ---
#include <iostream>

const int foo = 17;
foo = 42; // line 4

int main()
   std::cout << "Foo is " << foo << std::endl;
   return 0;
--- end of file.cpp ---

then you are making an even bigger mistake than the one we have been
discussing. The language does not allow assignments outside functions,
so the compiler will probably try to treat the assignment as another
declaration -- a C compiler will probably give an error saying that foo
is already declared while C++ will tell you that you have tried to
declare a variable without specifying its type (C allows you to omit
the type from a declaration of an int).

In exactly the same way you can't call functions at global scope except
to initialize variables, so -- back to your original post -- you can't
do this at all:

#include "MyInfoArrayClass.h"

const CMyInfoArrayClass g_Info;

The problem is NOT only that g_Info is const, but also that you are not
allowed to call Add outside a function.

I've just tried this with VS2005. I have the code:

--- test1.cpp ---
#include <vector>

std::vector<int> iv;
--- end of test1.cpp ---

and I get the following errors:

1>e:\dev\cpp\test\test\test1.cpp(4) : error C2143: syntax error : missing ';' before '.'
1>e:\dev\cpp\test\test\test1.cpp(4) : error C4430: missing type specifier - int assumed. Note: C++ does not
support default-int
1>e:\dev\cpp\test\test\test1.cpp(4) : error C2371: 'vi' : redefinition; different basic types
1> e:\dev\cpp\test\test\test1.cpp(3) : see declaration of 'vi'

The first two errors arise because the compiler is expecting a declaration,
and the code I've written doesn't look like one. C2143 says I can't have
a dot in an declaration and C4430 says I have to specify a type.

The third error arises because the compiler thinks that line 4 is trying to
be another declaration for iv, and it says that iv is already defined with a
different type.

Note that I get *exactly* the same errors if I define iv to be a const vector,
the compiler rejects the line before even noticing that I'm trying to change a

Note also that the code above compiles perfectly if I put it inside a

--- test2.cpp ---
#include <vector>

void bar()
  std::vector<int> iv;
--- end of test2.cpp ---

Now, of course, if I make iv a const vector the compiler will complain ... but
that is not your main problem.

There are a number of ways to approach what you're trying to do that will
allow you to get around this problem, but the approach you've tried so far
will not work.

The first question I would ask is: Does your global array really need to be
a C++ class, or could you use a C-style array (a POD type: Plain Old Data).

I don't know what the data structures are that you want to store, so I'll
make up a simple example of the sort of thing you seem to be talking about:

Say you have a system in which there are a number of commands, each identified
by some command number, and each having a short command string and 0 to 4
numeric parameters (for the sake of argument).

You could then define the structure for a command like this:

const int MaxCommandArgs = 4;

struct Command
   int commandNumber;
   const char * commandString;
   int nArgs;
   int args[MaxCommandArgs];

and you could create a global array of command structures like this:

const Command globalCommandArray[] =
   { 1, "Go", 1, { 12 } },
   { 42, "Stop", 0 },
   { 23, "Turn", 2, { -1, 20 } },
   { 24, "Turn", 2, { 1, 20 } },
   // last entry is dummy with command number 0 to mark the end of the array
   { 0 }

Note that in this example the array *CAN* be const, because all the
values are set at once when the array is initialized. This is all just
C code -- there's no fancy C++ processing here -- but it's simple and
it works for this example. The technique may also apply to your
more complex case, I don't know.

The disadvantage of such an array is that there can be no checking of the
values at compile time. If you wrote

   { 23, "Turn", 2, { -1, 20, 17 } },

The compiler wouldn't tell you that you had specified nArgs of 2 and then
provided 3 arguments.

If you really need to have a variable-sized array you can do something
similar to your example, but write a function to do the initializing:

CMyInfoArrayClass g_Info;
bool g_Info_SetUpOK = initialize_g_Info();

bool initialize_g_Info()
  return true;

When the program starts the code generated by the compiler will call
initialize_g_Info in order to get the initializer value for g_Info_SetUpOK.
The only reason that g_Info_SetUpOK exists is to force the compiler to
call initialize_g_Info at program startup (though if there was some
possibility that initialization could fail you could use it to check
later whether the setup had worked);

Note, though, that this only works if g_Info is NOT const.

If const-ness is important to you you could wrap the whole array in
a class that only allows read access to it:

class CommandSet
   CMyInfoArrayClass info;

   const CommandData & GetCommandData( index i )
   { return info[i]; }
You can then declare a single global instance of the CommandSet class:

CommandSet g_Info;

You can initialize all the members of your CMyInfoArrayClass in the
constructor of the CommandSet class (so the constructor would look a
bit like the initialize_g_Info function above) but the only access that
the rest of the program would have to the array would be through the
GetCommandData function which returns only a const reference ... so you
effectively have the const qualifier that you wanted even though the
array is not actually const.

I have read somewhere that it is an unwritten convention ...

Surely, if you read it then it can't have been unwritten ? <smile>

... that a C file should never go beyond 1000 lines except in very
rare occasions to ease debug and future manipulation.

The general rule is that a C++ source file should be as short as possible
but not more so. It may make sense, for example, to write the whole of
the implementation of a single class in a single source file -- even
though that may be a lot of code. On the other hand it may make sense to
write the functions controlling some aspects of the behaviour of a
particular class in a separate file from the rest of the class to save
recompiling the whole class every time a change is made.

I once worked on a C project where it was decided that we would write no
more than one function in each file. That led to a lot of silly little
files containing functions of only a few lines, some of which were only
called from one other place in the code. As each C file had to have its
own documentation file, and each had to be signed off by the manager
whenever there was a change, it added a lot to the administrative overhead
of the project.

As you gain more experience you will learn what "feels right".


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