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 35.13.2 about that.
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);
}
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