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>Chapter 33. <SPAN
CLASS="APPLICATION"
>ECPG</SPAN
> - Embedded <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> in C</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ECPG-CPP"
>33.13. <ACRONYM
CLASS="ACRONYM"
>C++</ACRONYM
> Applications</A
></H1
><P
> ECPG has some limited support for C++ applications. This section
describes some caveats.
</P
><P
> The <TT
CLASS="COMMAND"
>ecpg</TT
> 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 <TT
CLASS="FILENAME"
>.c</TT
> file. The header file
declarations of the library functions used by the C language chunks
that <TT
CLASS="COMMAND"
>ecpg</TT
> generates are wrapped
in <TT
CLASS="LITERAL"
>extern "C" { ... }</TT
> blocks when used under
C++, so they should work seamlessly in C++.
</P
><P
> In general, however, the <TT
CLASS="COMMAND"
>ecpg</TT
> 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.
</P
><P
> 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 <A
HREF="ecpg-cpp.html#ECPG-CPP-AND-C"
>Section 33.13.2</A
>
about that.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-CPP-SCOPE"
>33.13.1. Scope for Host Variables</A
></H2
><P
> The <TT
CLASS="COMMAND"
>ecpg</TT
> 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 <TT
CLASS="COMMAND"
>ecpg</TT
> preprocessor will not understand the
scope of the class member variables.
</P
><P
> For example, in the following case, the <TT
CLASS="COMMAND"
>ecpg</TT
>
preprocessor cannot find any declaration for the
variable <TT
CLASS="LITERAL"
>dbname</TT
> in the <TT
CLASS="LITERAL"
>test</TT
>
method, so an error will occur.
</P><PRE
CLASS="PROGRAMLISTING"
>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;
}
void Test::test()
{
EXEC SQL SELECT current_database() INTO :dbname;
printf("current_database = %s\n", dbname);
}
TestCpp::~TestCpp()
{
EXEC SQL DISCONNECT ALL;
}</PRE
><P>
This code will result in an error like this:
</P><PRE
CLASS="SCREEN"
><KBD
CLASS="USERINPUT"
>ecpg test_cpp.pgc</KBD
>
test_cpp.pgc:28: ERROR: variable "dbname" is not declared</PRE
><P>
</P
><P
> To avoid this scope issue, the <TT
CLASS="LITERAL"
>test</TT
> 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.
</P><PRE
CLASS="PROGRAMLISTING"
>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);
}</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-CPP-AND-C"
>33.13.2. C++ Application Development with External C Module</A
></H2
><P
> If you understand these technical limitations of
the <TT
CLASS="COMMAND"
>ecpg</TT
> 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.
</P
><P
> Three kinds of files have to be created: a C file
(<TT
CLASS="FILENAME"
>*.pgc</TT
>), a header file, and a C++ file:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="FILENAME"
>test_mod.pgc</TT
></DT
><DD
><P
> A sub-routine module to execute SQL commands embedded in C.
It is going to be converted
into <TT
CLASS="FILENAME"
>test_mod.c</TT
> by the preprocessor.
</P><PRE
CLASS="PROGRAMLISTING"
>#include "test_mod.h"
#include <stdio.h>
void
db_connect()
{
EXEC SQL CONNECT TO testdb1;
}
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;
}</PRE
><P>
</P
></DD
><DT
><TT
CLASS="FILENAME"
>test_mod.h</TT
></DT
><DD
><P
> A header file with declarations of the functions in the C
module (<TT
CLASS="FILENAME"
>test_mod.pgc</TT
>). It is included by
<TT
CLASS="FILENAME"
>test_cpp.cpp</TT
>. This file has to have an
<TT
CLASS="LITERAL"
>extern "C"</TT
> block around the declarations,
because it will be linked from the C++ module.
</P><PRE
CLASS="PROGRAMLISTING"
>#ifdef __cplusplus
extern "C" {
#endif
void db_connect();
void db_test();
void db_disconnect();
#ifdef __cplusplus
}
#endif</PRE
><P>
</P
></DD
><DT
><TT
CLASS="FILENAME"
>test_cpp.cpp</TT
></DT
><DD
><P
> The main code for the application, including
the <CODE
CLASS="FUNCTION"
>main</CODE
> routine, and in this example a
C++ class.
</P><PRE
CLASS="PROGRAMLISTING"
>#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;
}</PRE
><P>
</P
></DD
></DL
></DIV
><P>
</P
><P
> To build the application, proceed as follows. Convert
<TT
CLASS="FILENAME"
>test_mod.pgc</TT
> into <TT
CLASS="FILENAME"
>test_mod.c</TT
> by
running <TT
CLASS="COMMAND"
>ecpg</TT
>, and generate
<TT
CLASS="FILENAME"
>test_mod.o</TT
> by compiling
<TT
CLASS="FILENAME"
>test_mod.c</TT
> with the C compiler:
</P><PRE
CLASS="PROGRAMLISTING"
>ecpg -o test_mod.c test_mod.pgc
cc -c test_mod.c -o test_mod.o</PRE
><P>
</P
><P
> Next, generate <TT
CLASS="FILENAME"
>test_cpp.o</TT
> by compiling
<TT
CLASS="FILENAME"
>test_cpp.cpp</TT
> with the C++ compiler:
</P><PRE
CLASS="PROGRAMLISTING"
>c++ -c test_cpp.cpp -o test_cpp.o</PRE
><P>
</P
><P
> Finally, link these object files, <TT
CLASS="FILENAME"
>test_cpp.o</TT
>
and <TT
CLASS="FILENAME"
>test_mod.o</TT
>, into one executable, using the C++
compiler driver:
</P><PRE
CLASS="PROGRAMLISTING"
>c++ test_cpp.o test_mod.o -lecpg -o test_cpp</PRE
><P>
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