View Compilation Error

0_0_9689680_1988.cpp: In function `int main()':
0_0_9689680_1988.cpp:22: error: `A' undeclared (first use this function)
0_0_9689680_1988.cpp:22: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_9689680_1988.cpp:22: error: `B' undeclared (first use this function)