本教程将使用libpqxx库，它是PostgreSQL的官方C ++客户端API。 libpqxx的源代码在BSD许可下可用，因此您可以免费下载，将其传递给他人，进行更改，出售，将其包含在自己的代码中，并与选择的任何人共享您的更改。
最新版本的libpqxx可以从下载libpqxx链接下载(Libpqxx下载)。因此，请下载最新版本并按照以下步骤-
wget http://pqxx.org/download/software/libpqxx/libpqxx-4.0.tar.gz tar xvfz libpqxx-4.0.tar.gz cd libpqxx-4.0 ./configure make make install
在开始使用C / C ++ PostgreSQL接口之前，请在PostgreSQL安装目录中找到 pg_hba.conf 文件，并添加以下行-
# IPv4 local connections: host all all 127.0.0.1/32 md5
如果postgres服务器没有运行，可以使用以下命令启动/重新启动postgres服务器-
[root@host]# service postgresql restart Stopping postgresql service: [ OK ] Starting postgresql service: [ OK ]
下面是重要的接口例程，它可以满足您从C/C++程序中使用PostgreSQL数据库的需求。如果您正在寻找一个更复杂的应用程序，那么您可以查看libpqxx官方文档，也可以使用商业上可用的api。
| S. No. | API & 描述 |
|---|---|
| 1 | pqxx::connection C( const std::string & dbstring ) 这是一个typedef，将用于连接到数据库。这里，dbstring提供连接到数据库所需的参数,例如: dbname = testdb user = postgres password=pass123 hostaddr=127.0.0.1 port=5432. 如果连接成功建立，那么它创建了C与连接对象，提供各种有用的函数公共函数。 |
| 2 | C.is_open() is_open()方法是连接对象的公共方法，并返回布尔值。如果连接是活动的，则此方法返回true，否则返回false。 |
| 3 | C.disconnect() 此方法用于断开已打开的数据库连接。 |
| 4 | pqxx::work W( C ) 这是一个 typedef，用于使用连接 C 创建事务对象，最终用于在事务模式下执行 SQL 语句。 如果事务对象创建成功，那么它将被分配给变量W，该变量将用于访问与事务对象相关的公共方法。 |
| 5 | W.exec(const std::string & sql) 这个来自事务对象的公共方法将用于执行SQL语句。 |
| 6 | W.commit() 来自事务对象的这个公共方法将用于提交事务。 |
| 7 | W.abort() 事务对象的这个公共方法将用于回滚事务。 |
| 8 | pqxx::nontransaction N( C ) 这是一个typedef，它将用于使用连接C创建非事务性对象，最终将用于在非事务模式下执行SQL语句。 如果事务对象创建成功，那么它将被分配给变量N，该变量将用于访问与非事务对象相关的公共方法。 |
| 9 | N.exec(const std::string & sql) 这个来自非事务对象的公共方法将用于执行SQL语句并返回一个result对象，它实际上是一个保存所有返回记录的interator。 |
下面的C代码段显示了如何连接到在端口5432的本地计算机上运行的现有数据库。这里，我用反斜杠\表示行的延续。
#include <iostream>
#include <pqxx/pqxx>
using namespace std;
using namespace pqxx;
int main(int argc, char* argv[]) {
try {
connection C("dbname = testdb user = postgres password = cohondob \
hostaddr = 127.0.0.1 port = 5432");
if (C.is_open()) {
cout << "Opened database successfully: " << C.dbname() << endl;
} else {
cout << "Can't open database" << endl;
return 1;
}
C.disconnect ();
} catch (const std::exception &e) {
cerr << e.what() << std::endl;
return 1;
}
}现在，让我们编译并运行上面的程序，以连接到我们的数据库 testdb，该数据库已经在您的模式中可用，可以使用用户 postgres 和密码 pass123访问。
您可以根据数据库设置使用用户ID和密码。记住保持-lpqxx和-lpq的给定顺序!否则，连接器将会认为缺少名称而以“PQ”开头的函数。
$g++ test.cpp -lpqxx -lpq $./a.out Opened database successfully: testdb
以下C代码段将用于在先前创建的数据库中创建表-
#include <iostream>
#include <pqxx/pqxx>
using namespace std;
using namespace pqxx;
int main(int argc, char* argv[]) {
char * sql;
try {
connection C("dbname = testdb user = postgres password = cohondob \
hostaddr = 127.0.0.1 port = 5432");
if (C.is_open()) {
cout << "Opened database successfully: " << C.dbname() << endl;
} else {
cout << "Can't open database" << endl;
return 1;
}
/* 创建SQL语句 */
sql = "CREATE TABLE COMPANY(" \
"ID INT PRIMARY KEY NOT NULL," \
"NAME TEXT NOT NULL," \
"AGE INT NOT NULL," \
"ADDRESS CHAR(50)," \
"SALARY REAL );";
/* 创建事务对象. */
work W(C);
/* 执行SQL查询 */
W.exec( sql );
W.commit();
cout << "Table created successfully" << endl;
C.disconnect ();
} catch (const std::exception &e) {
cerr << e.what() << std::endl;
return 1;
}
return 0;
}当编译并执行上述给定程序时，它将在testdb数据库中创建COMPANY表，并显示以下语句-
Opened database successfully: testdb Table created successfully
下面的C代码段显示了如何在上面示例中创建的company表中创建记录-
#include <iostream>
#include <pqxx/pqxx>
using namespace std;
using namespace pqxx;
int main(int argc, char* argv[]) {
char * sql;
try {
connection C("dbname = testdb user = postgres password = cohondob \
hostaddr = 127.0.0.1 port = 5432");
if (C.is_open()) {
cout << "Opened database successfully: " << C.dbname() << endl;
} else {
cout << "Can't open database" << endl;
return 1;
}
/* 创建 SQL 语句 */
sql = "INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) " \
"VALUES (1, 'Paul', 32, 'California', 20000.00 ); " \
"INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) " \
"VALUES (2, 'Allen', 25, 'Texas', 15000.00 ); " \
"INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)" \
"VALUES (3, 'Teddy', 23, 'Norway', 20000.00 );" \
"INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)" \
"VALUES (4, 'Mark', 25, 'Rich-Mond ', 65000.00 );";
/* 创建事务对象. */
work W(C);
/* 执行SQL查询 */
W.exec( sql );
W.commit();
cout << "Records created successfully" << endl;
C.disconnect ();
} catch (const std::exception &e) {
cerr << e.what() << std::endl;
return 1;
}
return 0;
}当编译并执行上述给定程序时，它将在COMPANY表中创建给定的记录，并显示以下两行-
Opened database successfully: testdb Records created successfully
下面的C代码段展示了我们如何从上面示例中创建的COMPANY表中获取和显示记录
#include <iostream>
#include <pqxx/pqxx>
using namespace std;
using namespace pqxx;
int main(int argc, char* argv[]) {
char * sql;
try {
connection C("dbname = testdb user = postgres password = cohondob \
hostaddr = 127.0.0.1 port = 5432");
if (C.is_open()) {
cout << "Opened database successfully: " << C.dbname() << endl;
} else {
cout << "Can't open database" << endl;
return 1;
}
/* 创建 SQL 语句 */
sql = "SELECT * from COMPANY";
/* 创建非事务对象. */
nontransaction N(C);
/* 执行 SQL 查询 */
result R( N.exec( sql ));
/* 列出所有的记录 */
for (result::const_iterator c = R.begin(); c != R.end(); ++c) {
cout << "ID = " << c[0].as<int>() << endl;
cout << "Name = " << c[1].as<string>() << endl;
cout << "Age = " << c[2].as<int>() << endl;
cout << "Address = " << c[3].as<string>() << endl;
cout << "Salary = " << c[4].as<float>() << endl;
}
cout << "Operation done successfully" << endl;
C.disconnect ();
} catch (const std::exception &e) {
cerr << e.what() << std::endl;
return 1;
}
return 0;
}当编译并执行上述给定程序时，它将产生以下结果-
Opened database successfully: testdb ID = 1 Name = Paul Age = 32 Address = California Salary = 20000 ID = 2 Name = Allen Age = 25 Address = Texas Salary = 15000 ID = 3 Name = Teddy Age = 23 Address = Norway Salary = 20000 ID = 4 Name = Mark Age = 25 Address = Rich-Mond Salary = 65000 Operation done successfully
下面的 c 代码段显示了如何使用 UPDATE 语句更新任何记录，然后从 COMPANY 表-获取并显示更新的记录
#include <iostream>
#include <pqxx/pqxx>
using namespace std;
using namespace pqxx;
int main(int argc, char* argv[]) {
char * sql;
try {
connection C("dbname = testdb user = postgres password = cohondob \
hostaddr = 127.0.0.1 port = 5432");
if (C.is_open()) {
cout << "Opened database successfully: " << C.dbname() << endl;
} else {
cout << "Can't open database" << endl;
return 1;
}
/* 创建事务对象. */
work W(C);
/* Create SQL UPDATE statement */
sql = "UPDATE COMPANY set SALARY = 25000.00 where ID=1";
/* Execute SQL query */
W.exec( sql );
W.commit();
cout << "Records updated successfully" << endl;
/* 创建 SQL SELECT 语句 */
sql = "SELECT * from COMPANY";
/* 创建一个非事务对象. */
nontransaction N(C);
/* 执行 SQL 查询 */
result R( N.exec( sql ));
/* 列出所有的记录 */
for (result::const_iterator c = R.begin(); c != R.end(); ++c) {
cout << "ID = " << c[0].as<int>() << endl;
cout << "Name = " << c[1].as<string>() << endl;
cout << "Age = " << c[2].as<int>() << endl;
cout << "Address = " << c[3].as<string>() << endl;
cout << "Salary = " << c[4].as<float>() << endl;
}
cout << "Operation done successfully" << endl;
C.disconnect ();
} catch (const std::exception &e) {
cerr << e.what() << std::endl;
return 1;
}
return 0;
}当上述给定的程序被编译和执行时，它将产生以下结果-
Opened database successfully: testdb Records updated successfully ID = 2 Name = Allen Age = 25 Address = Texas Salary = 15000 ID = 3 Name = Teddy Age = 23 Address = Norway Salary = 20000 ID = 4 Name = Mark Age = 25 Address = Rich-Mond Salary = 65000 ID = 1 Name = Paul Age = 32 Address = California Salary = 25000 Operation done successfully
下面的 C 代码段显示了如何使用 DELETE 语句删除任何记录，然后从 COMPANY 表-中获取并显示剩余的记录
#include <iostream>
#include <pqxx/pqxx>
using namespace std;
using namespace pqxx;
int main(int argc, char* argv[]) {
char * sql;
try {
connection C("dbname = testdb user = postgres password = cohondob \
hostaddr = 127.0.0.1 port = 5432");
if (C.is_open()) {
cout << "Opened database successfully: " << C.dbname() << endl;
} else {
cout << "Can't open database" << endl;
return 1;
}
/* 创建事务对象. */
work W(C);
/* 创建 SQL DELETE 语句 */
sql = "DELETE from COMPANY where ID = 2";
/* 执行 SQL 查询 */
W.exec( sql );
W.commit();
cout << "Records deleted successfully" << endl;
/* Create SQL SELECT statement */
sql = "SELECT * from COMPANY";
/* 创建非事务对象. */
nontransaction N(C);
/* 执行 SQL 查询 */
result R( N.exec( sql ));
/* 列出所有的记录 */
for (result::const_iterator c = R.begin(); c != R.end(); ++c) {
cout << "ID = " << c[0].as<int>() << endl;
cout << "Name = " << c[1].as<string>() << endl;
cout << "Age = " << c[2].as<int>() << endl;
cout << "Address = " << c[3].as<string>() << endl;
cout << "Salary = " << c[4].as<float>() << endl;
}
cout << "Operation done successfully" << endl;
C.disconnect ();
} catch (const std::exception &e) {
cerr << e.what() << std::endl;
return 1;
}
return 0;
}当编译并执行上述给定程序时，它将产生以下结果-
Opened database successfully: testdb Records deleted successfully ID = 3 Name = Teddy Age = 23 Address = Norway Salary = 20000 ID = 4 Name = Mark Age = 25 Address = Rich-Mond Salary = 65000 ID = 1 Name = Paul Age = 32 Address = California Salary = 25000 Operation done successfully