13. Interfacing to Other Languages

The facilities in Annex B of the Ada Reference Manual are fully implemented in GNAT, and in addition, a full interface to C++ is provided.

13.1. Interfacing to C

Interfacing to C with GNAT can use one of two approaches:

  • The types in the package Interfaces.C may be used.

  • Standard Ada types may be used directly. This may be less portable to other compilers, but will work on all GNAT compilers, which guarantee correspondence between the C and Ada types.

Pragma Convention C may be applied to Ada types, but mostly has no effect, since this is the default. The following table shows the correspondence between Ada scalar types and the corresponding C types.

Ada Type

C Type






signed char




long long








This is the longest floating-point type supported by the hardware.

Additionally, there are the following general correspondences between Ada and C types:

  • Ada enumeration types map to C enumeration types directly if pragma Convention C is specified, which causes them to have a length of 32 bits, except for boolean types which map to C99 bool and for which the length is 8 bits. Without pragma Convention C, Ada enumeration types map to 8, 16, or 32 bits (i.e., C types signed char, short, int, respectively) depending on the number of values passed. This is the only case in which pragma Convention C affects the representation of an Ada type.

  • Ada access types map to C pointers, except for the case of pointers to unconstrained types in Ada, which have no direct C equivalent.

  • Ada arrays map directly to C arrays.

  • Ada records map directly to C structures.

  • Packed Ada records map to C structures where all members are bit fields of the length corresponding to the type'Size value in Ada.

13.2. Interfacing to C++

The interface to C++ makes use of the following pragmas, which are primarily intended to be constructed automatically using a binding generator tool, although it is possible to construct them by hand.

Using these pragmas it is possible to achieve complete inter-operability between Ada tagged types and C++ class definitions. See Implementation Defined Pragmas, for more details.

pragma CPP_Class ([Entity =>] LOCAL_NAME)

The argument denotes an entity in the current declarative region that is declared as a tagged or untagged record type. It indicates that the type corresponds to an externally declared C++ class type, and is to be laid out the same way that C++ would lay out the type.

Note: Pragma CPP_Class is currently obsolete. It is supported for backward compatibility but its functionality is available using pragma Import with Convention = CPP.

pragma CPP_Constructor ([Entity =>] LOCAL_NAME)

This pragma identifies an imported function (imported in the usual way with pragma Import) as corresponding to a C++ constructor.

A few restrictions are placed on the use of the Access attribute in conjunction with subprograms subject to convention CPP: the attribute may be used neither on primitive operations of a tagged record type with convention CPP, imported or not, nor on subprograms imported with pragma CPP_Constructor.

In addition, C++ exceptions are propagated and can be handled in an others choice of an exception handler. The corresponding Ada occurrence has no message, and the simple name of the exception identity contains Foreign_Exception. Finalization and awaiting dependent tasks works properly when such foreign exceptions are propagated.

It is also possible to import a C++ exception using the following syntax:

LOCAL_NAME : exception;
pragma Import (Cpp,
  [Entity =>] LOCAL_NAME,
  [External_Name =>] static_string_EXPRESSION);

The External_Name is the name of the C++ RTTI symbol. You can then cover a specific C++ exception in an exception handler.

13.3. Interfacing to COBOL

Interfacing to COBOL is achieved as described in section B.4 of the Ada Reference Manual.

13.4. Interfacing to Fortran

Interfacing to Fortran is achieved as described in section B.5 of the Ada Reference Manual. The pragma Convention Fortran, applied to a multi-dimensional array causes the array to be stored in column-major order as required for convenient interface to Fortran.

13.5. Interfacing to non-GNAT Ada code

It is possible to specify the convention Ada in a pragma Import or pragma Export. However this refers to the calling conventions used by GNAT, which may or may not be similar enough to those used by some other Ada 83 / Ada 95 / Ada 2005 compiler to allow interoperation.

If arguments types are kept simple, and if the foreign compiler generally follows system calling conventions, then it may be possible to integrate files compiled by other Ada compilers, provided that the elaboration issues are adequately addressed (for example by eliminating the need for any load time elaboration).

In particular, GNAT running on VMS is designed to be highly compatible with the DEC Ada 83 compiler, so this is one case in which it is possible to import foreign units of this type, provided that the data items passed are restricted to simple scalar values or simple record types without variants, or simple array types with fixed bounds.