34.13. C++ Applications
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## 34.13. C++ Applications #

ECPG has some limited support for C++ applications. This section describes some caveats.

The ecpg preprocessor takes an input file written in C (or something like C) and embedded SQL commands, converts the embedded SQL commands into C language chunks, and finally generates a `.c` file. The header file declarations of the library functions used by the C language chunks that ecpg generates are wrapped in `extern "C" { ... }` blocks when used under C++, so they should work seamlessly in C++.

In general, however, the ecpg preprocessor only understands C; it does not handle the special syntax and reserved words of the C++ language. So, some embedded SQL code written in C++ application code that uses complicated features specific to C++ might fail to be preprocessed correctly or might not work as expected.

A safe way to use the embedded SQL code in a C++ application is hiding the ECPG calls in a C module, which the C++ application code calls into to access the database, and linking that together with the rest of the C++ code. See Section 34.13.2 about that.

### 34.13.1. Scope for Host Variables #

The ecpg preprocessor understands the scope of variables in C. In the C language, this is rather simple because the scopes of variables is based on their code blocks. In C++, however, the class member variables are referenced in a different code block from the declared position, so the ecpg preprocessor will not understand the scope of the class member variables.

For example, in the following case, the ecpg preprocessor cannot find any declaration for the variable `dbname` in the `test` method, so an error will occur.

    class TestCpp
    {
        EXEC SQL BEGIN DECLARE SECTION;
        char dbname[1024];
        EXEC SQL END DECLARE SECTION;

      public:
        TestCpp();
        void test();
        ~TestCpp();
    };

    TestCpp::TestCpp()
    {
        EXEC SQL CONNECT TO testdb1;
        EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
    }

    void Test::test()
    {
        EXEC SQL SELECT current_database() INTO :dbname;
        printf("current_database = %s\n", dbname);
    }

    TestCpp::~TestCpp()
    {
        EXEC SQL DISCONNECT ALL;
    }

This code will result in an error like this:

    ecpg test_cpp.pgc
    test_cpp.pgc:28: ERROR: variable "dbname" is not declared

To avoid this scope issue, the `test` method could be modified to use a local variable as intermediate storage. But this approach is only a poor workaround, because it uglifies the code and reduces performance.

    void TestCpp::test()
    {
        EXEC SQL BEGIN DECLARE SECTION;
        char tmp[1024];
        EXEC SQL END DECLARE SECTION;

        EXEC SQL SELECT current_database() INTO :tmp;
        strlcpy(dbname, tmp, sizeof(tmp));

        printf("current_database = %s\n", dbname);
    }

### 34.13.2. C++ Application Development with External C Module #

If you understand these technical limitations of the ecpg preprocessor in C++, you might come to the conclusion that linking C objects and C++ objects at the link stage to enable C++ applications to use ECPG features could be better than writing some embedded SQL commands in C++ code directly. This section describes a way to separate some embedded SQL commands from C++ application code with a simple example. In this example, the application is implemented in C++, while C and ECPG is used to connect to the PostgreSQL server.

Three kinds of files have to be created: a C file (`*.pgc`), a header file, and a C++ file:

`test_mod.pgc`

  A sub-routine module to execute SQL commands embedded in C. It is going to be converted into `test_mod.c` by the preprocessor.

      #include "test_mod.h"
      #include <stdio.h>

      void
      db_connect()
      {
          EXEC SQL CONNECT TO testdb1;
          EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
      }

      void
      db_test()
      {
          EXEC SQL BEGIN DECLARE SECTION;
          char dbname[1024];
          EXEC SQL END DECLARE SECTION;

          EXEC SQL SELECT current_database() INTO :dbname;
          printf("current_database = %s\n", dbname);
      }

      void
      db_disconnect()
      {
          EXEC SQL DISCONNECT ALL;
      }

`test_mod.h`

  A header file with declarations of the functions in the C module (`test_mod.pgc`). It is included by `test_cpp.cpp`. This file has to have an `extern "C"` block around the declarations, because it will be linked from the C++ module.

      #ifdef __cplusplus
      extern "C" {
      #endif

      void db_connect();
      void db_test();
      void db_disconnect();

      #ifdef __cplusplus
      }
      #endif

`test_cpp.cpp`

  The main code for the application, including the `main` routine, and in this example a C++ class.

      #include "test_mod.h"

      class TestCpp
      {
        public:
          TestCpp();
          void test();
          ~TestCpp();
      };

      TestCpp::TestCpp()
      {
          db_connect();
      }

      void
      TestCpp::test()
      {
          db_test();
      }

      TestCpp::~TestCpp()
      {
          db_disconnect();
      }

      int
      main(void)
      {
          TestCpp *t = new TestCpp();

          t->test();
          return 0;
      }

To build the application, proceed as follows. Convert `test_mod.pgc` into `test_mod.c` by running ecpg, and generate `test_mod.o` by compiling `test_mod.c` with the C compiler:

    ecpg -o test_mod.c test_mod.pgc
    cc -c test_mod.c -o test_mod.o

Next, generate `test_cpp.o` by compiling `test_cpp.cpp` with the C++ compiler:

    c++ -c test_cpp.cpp -o test_cpp.o

Finally, link these object files, `test_cpp.o` and `test_mod.o`, into one executable, using the C++ compiler driver:

    c++ test_cpp.o test_mod.o -lecpg -o test_cpp

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