PostgreSQL 9.6.24 Documentation
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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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