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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >C++ Applications</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.24 Documentation" HREF="index.html"><LINK REL="UP" TITLE="ECPG - Embedded SQL in C" HREF="ecpg.html"><LINK REL="PREVIOUS" TITLE="Large Objects" HREF="ecpg-lo.html"><LINK REL="NEXT" TITLE="Embedded SQL Commands" HREF="ecpg-sql-commands.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2017-11-06T22:43:11"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.24 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Large Objects" HREF="ecpg-lo.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="ecpg.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 33. <SPAN CLASS="APPLICATION" >ECPG</SPAN > - Embedded <ACRONYM CLASS="ACRONYM" >SQL</ACRONYM > in C</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Embedded SQL Commands" HREF="ecpg-sql-commands.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><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> </P ></DIV ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="ecpg-lo.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="ecpg-sql-commands.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Large Objects</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="ecpg.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Embedded SQL Commands</TD ></TR ></TABLE ></DIV ></BODY ></HTML >