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Why did I call the math library, or "PI", an undeclared identifier, appear? Ask god for advice.
The macro in the header file math.h is defined as M_PI.

# Define M_PI 3. 14 15926535

The C++ language itself is so complicated that it is even difficult for human beings to understand its semantics. To make matters worse, due to the complexity of C++, the compilation system of C++ is very difficult to write. Even available compilers have many problems, most of which are difficult to find.

Because of its complexity, it is difficult to guarantee the correctness of complex C++ programs. Some people have also pointed out some defects, such as not supporting multithreading primitives. But so many celebrities have raised so many defects, which shows that C++ is widely used and successful.

Also known as thermonuclear reaction). Atomic bomb and man-made now