A Tour of NTL: Traditional and ISO Modes

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A Tour of NTL: Traditional and ISO Modes

As of version 4.1, NTL can be compiled and used in one of two modes: Traditional or ISO. As of NTL version 5.4, ISO mode is the default.

To revert to traditional mode, you can pass `NTL_STD_CXX=off` as an argument to the configuration script when installing NTL on a Unix or Unix-like system, which will unset the flag `NTL_STD_CXX` in the `config.h` file. Alternatively (and especially on non-Unix systems), you can unset this flag by hand by editing the the `config.h` file.

In Traditional mode, the NTL header files include the traditional `C++` header files `<stdlib.h>`, `<math.h>`, and `<iostream.h>`. These files declare a number of names (functions, types, etc.) in the global namespace. Additionally, the NTL header files declare a number of names, also in the global namespace.

In ISO mode, three things change:

1. NTL namespace: The NTL header files wrap all NTL names in a namespace, called `NTL`.

2. New header files: The NTL header files include the new `C++` header files `<cstdlib>`, `<cmath>`, and `<iostream>`. These new header files are essentially the same as the traditional ones, except that all the the names are declared in a namespace called `std`.

3. Nothrow new: The NTL implementation files use the `nothrow` version of `new`.

If your complier is not up to date, but you want some of the benefits of Standard `C++`, you can set the partial standard flags to get any subset of the above three changes:

1. `NTL_PSTD_NNS`: NTL namespace
2. `NTL_PSTD_NHF`: New header files
3. `NTL_PSTD_NTN`: Nothrow new

You can set these flags either by using the configuration script (only on Unix-like systems), or by editing the `config.h` file. For example, to just wrap NTL in a namepsace, just pass `NTL_PSTD_NNS=on` as an argument to the configuration script when installing NTL. However, make sure you also turn off the `NTL_STD_CXX` flag; otherwise, these have no effect.

Especially when combining NTL with other libraries, the `NTL_PSTD_NNS` flag may be particularly useful in avoiding name clashes, even if your compiler has just a rudimentary implementation of namespaces.

NTL will remain usable in Traditional mode indefinitely, assuming compilers maintain reasonable backward compatibilty with pre-standard `C++` conventions for header files; however, if you want to program for the future, it is recommended to use ISO mode. The partial ISO modes are not highly recommended; they are mainly intended as a stop-gap measure while we wait for decent standard-conforming `C++` compilers to become available.

### A crash course on namespaces

As already mentioned, the main difference between Traditional and ISO mode is that in ISO mode, all names are wrapped in namespaces. Namespaces are a feature that was introduced in the new `C++` standard. One can declare names (functions, types, etc.) inside a namespace. By default, such names are not visible outside the namespace without explicit qualification.

The main advantage of namespaces is that it solves the namespace pollution problem: if two libraries define the same name in two inconsistent ways, it is very difficult, if not impossible, to combine these two libraries in the same program.

The traditional way of avoiding such problems in languages like `C` is for a library designer to attach a prefix specific to that library to all names. This works, but makes for ugly code. The function overloading mechanism in `C++` eases the problem a bit, but is still not a complete solution.

The new namespace feature in `C++` provides a reasonably complete and elegant solution to the namespace pollution problem. It is one of the nicest and most important recent additions to the `C++` language.

Here is a simple example to illustrate namespaces.

    namespace N {
       void f(int);
       void g(int);
       int x;
    }

    int x;

    void h()
    {
       x = 1;    // the global x
       N::x = 0; // the x in namespace N
       N::f(0);  // the f in namespace N
       g(1);     // error -- g is not visible here
    }

All of this explicit qualification business can be a bit tedious. The easiest way to avoid this tedium is to use what is called a using directive, which effectively makes all names declared within a namespace visible in the global scope. Here is a variation on the previous example, with a using directive.

    namespace N {
       void f(int);
       void g(int);
       int x;
    }

    int x;

    using namespace N;

    void h()
    {
       x = 1;    // error -- ambiguous: the global x or the x in namespace N?
       ::x = 1;  // the global x
       N::x = 0; // the x in namespace N
       N::f(0);  // the f in namespace N
       f(0);     // OK -- N::f(int) is visible here
       g(1);     // OK -- N::g(int) is visible here
    }

Here is another example.

    namespace N1 {
       int x;
       void f(int);
       void g(int);
    }

    namespace N2 {
       int x;
       int y;
       void f(double);
       void g(int);
    }

    using namespace N1;
    using namespace N2;

    void h()
    {
       x = 1;     // error -- ambiguous: N1::x or N2::x?
       N1::x = 1; // OK
       N2::x = 1; // OK
       y = 1;     // OK  -- this is N2::y
       g(0);      // error -- ambiguous: N1::g(int) or N2::g(int)?
       f(0);      // OK -- N1::f(int), because it is the "best" match
       f(0.0);    // OK  -- N2::f(double), because it is the "best" match
    }

This example illustrates the interaction between using declarations and function overloading resolution. If several overloaded versions of a function are visible, it is not necessarily ambiguous: the usual overload resolution procedure is applied, and if there is a unique "best" match, then there is no ambiguity.

The examples presented here do not illustrate all of the features and nuances of namespaces. For this, you are referred to a `C++` book.

### Namespaces and NTL

In ISO mode, the standard library is "wrapped" in namespace `std`, and NTL is "wrapped" in namespace `NTL`. Thus, the header file `<NTL/ZZ.h>` in ISO mode looks something like this:

    namespace NTL {

       // ...

       class ZZ { /* ... */ };

       // ...

       ZZ operator+(const ZZ& a, const ZZ& b);
       ZZ operator*(const ZZ& a, const ZZ& b);

       std::istream& operator>>(std::istream& s, ZZ& x);
       std::ostream& operator<<(std::ostream& s, const ZZ& a);

       // ...

    }

Therefore, one must explicitly qualify all names, or use appropriate using directives. Here is how one could write the first example of the tour in ISO mode.

    #include <NTL/ZZ.h>

    int main()
    {
       NTL::ZZ a, b, c;

       std::cin >> a;
       std::cin >> b;
       c = (a+1)*(b+1);
       std::cout << c << "\n";
    }

Notice how everything is explicitly qualified. Actually, the input/output operators `<<` and `>>`, and the arithmetic operators `+` and `*` are not explicitly qualified, but rather, the compiler finds them through a gimmick called Koenig Lookup, which will look for functions (and operators) declared in namespace `NTL`, because the type of the argument (`ZZ`) is a class declared in that namespace.

Even with Koenig Lookup, explicit qualification can be a bit tedious. Here is the same example, this time with using directives.

    #include <NTL/ZZ.h>

    using namespace NTL;
    using namespace std;

    int main()
    {
       ZZ a, b, c;

       cin >> a;
       cin >> b;
       c = (a+1)*(b+1);
       cout << c << "\n";
    }

To write NTL client code that will compile smoothly in either Traditional or ISO mode, one simply does the following:

    #include <NTL/ZZ.h>

    NTL_CLIENT

    int main()
    {
       ZZ a, b, c;

       cin >> a;
       cin >> b;
       c = (a+1)*(b+1);
       cout << c << "\n";
    }

Here, `NTL_CLIENT` is a macro defined by NTL that expands into zero, one, or two appropriate using directives, depending on the settings of `NTL_STD_CXX`, `NTL_PSTD_NNS`, and `NTL_PSTD_NHF`. Alternatively, instead of using the `NTL_CLIENT` macro, you can write:

    #if (defined(NTL_PSTD_NNS) || defined(NTL_STD_CXX))
       using namespace NTL;
    #endif

    #if (defined(NTL_PSTD_NHF) || defined(NTL_STD_CXX))
       using namespace std;
    #endif

Typically, when writing a program that uses NTL, you can simply insert the `NTL_CLIENT` as above, and forget about all this namespace nonsense. However, if you are combining libraries, you may have to disambiguate things from time to time.

The Standard `C++` library is huge. If you just use `<iostream>`, you should not have any ambiguous names. However, there are some potential ambiguities in the STL (Standard Template Library) part of the library. One that I know of is the template class `negate` defined in `<functional>`, which conflicts with the NTL function `negate`. With namespaces, there should be no problem, unless the client code explicitly uses `negate`, in which case you will have to explicitly qualify `negate` to tell the compiler which `negate` you mean, either `std::negate` or `NTL::negate`.

NTL also explicitly defines various versions of `min` and `max` functions. Template versions of these functions are also defined in the standard library component `<algorithm>`. Because of the way the function overload resolution mechanism works, the "right" version of `min` or `max` should always be chosen, without any need for explicit qualification.

There may be other possible ambiguities between the standard library and NTL, but if they arise, they are easily fixed through explicit qualification.

### Some global names

It is not quite true that all names declared in NTL header files are wrapped in namespace NTL. There are two classes of exceptions:

- All names that start with the prefix "`NTL_`" are in fact macros. There are a number of documented and undocumented such macros. Note that any name with this prefix is a macro and all macros start with this prefix.

- There are also a number of undocumented names that start with the prefix "`_ntl_`". These are not macros, but rather are names of functions, types, etc., that are declared in the global namespace. Any name with this prefix is in the global namespace, and all names in the global namespace start with this prefix. All functions with `"C"` linkage have this prefix.

Thus, NTL "owns" all names starting with "`NTL_`" or "`_ntl_`"; users of NTL should avoid names with these prefixes.

### Further technicalities

Another thing to be aware of is that there are some small, annoying differences between the old standard `C` include files `<stdlib.h>` and `<math.h>`, and the new `C++` include files `<cstdlib>` and `<cmath>`, above and beyond the namespace wrapping. Specifically, the new header files declare several overloaded versions of some functions. For example, in the old header files, there was one function

       int abs(int);

Now there are several, including:

       int abs(int);
       long abs(long);
       float abs(float);
       double abs(double);
       long double abs(long double);

Also, functions like `log` and `sqrt` are also overloaded. So instead of just

       double log(double);

there are

       float log(float);
       double log(double);
       long double log(long double);

This can lead to compile-time errors in some old codes, such as:

       double log_2 = log(2);

With the old header files, the `int` value 2 would have been converted to a `double`, and the function

       double log(double);

would have been called.

With the new header files, the compiler would raise an error, because the function call is now ambiguous.

Of course, the fix is trivial:

       double log_2 = log(2.0);

This will compile correctly with either old or new header files.

Don't you just love the ISO?

### A note on documentation

The "`.txt`" files documenting NTL's modules still reflect NTL's Traditional mode. There should be no confusion in interpretting the meaning in ISO mode. Just remember: all of NTL is wrapped in namespace `NTL`, and the standard library is wrapped in namespace `std`.

### Further changes in NTL version 4.1

The ISO Standard for `C++` is not compatible with the language defined in the second edition of Stroustrup's `C++` book. This is in fact quite annoying. Besides introducing namespaces, several modifications were made in version 4.1 that will allow NTL to be compiled smoothly under either the old or the new definition of the language (or any reasonable approximation thereof). These changes do not affect the (documented) NTL interface, and so version 4.1 should be backward compatible.

Here is a summary of the other changes:

- Got rid of all `friend` functions. It turns out that new `C++` and old `C++` disagree quite strongly about the semantics of a `friend` function declaration. In getting rid of these, I also made a number of fields public which used to be private, but to prevent accidental misuse, I gave them strange names (e.g., the previously private member `rep` in class `ZZ_p` is now the public member `_ZZ_p__rep`).

  This change is effective in both Traditional and ISO modes.

  In my view, the ISO committee really committed an act of sabotage here. Now the `friend` mechanism is much more awkward than before, which makes the use of private members more awkward, which simply encourages programmers (like me) to avoid them altogether.

- When `NTL_STD_CXX` or `NTL_PSTD_NTN` are set, all calls to `new` have been replaced by `new(std::nothrow)`.

  The ISO committee also committed an act of sabotage when they changed the semantics of the memory allocation operator `new`. In old `C++`, a memory allocation error simply returned a null pointer; in new `C++` an exception is thrown. The old semantics are available via `new(std::nothrow)`.

  You may of course use NTL in Traditional mode with a compiler that implements the new semantics for `new`. In this case, if the memory allocation fails, an exception will be thrown, and assuming you don't catch it, you will simply get an error message that is less informative than the one NTL would have printed. Also, your compiler may have a backward compatatibilty flag to use the old `new` semantics.

- Various and sundry other small changes, such as fixing occurrences of the the "`log(2)`" problem mentioned above.

### Standard C++ and the Real World

The first C++ standard was set in 1998, with some revisions in 2003. As I write this update in 2013, I believe it is safe to say that most compileres released in the last few years do a pretty good job of implementing the standard.

However, a new revision to tne standard appeared in 2011. This new revision contains many new language and library features. Surely, it will be a number of years until support for all these new feautures will be uniformly available.