1.4.2.1 Use assert to find bugs early

• bugs should be found as early as possible. There are two basic methods to find bugs early: assert and unit test.
• Idea of assert is to make "an unnormal" can be spotted immediately, or this "an nunormal" will cause error in other place, then It’s a little difficult to trace back source.
   fun(char* p){ 
      assert(p!=nullptr); 
      ...... // a lot of codes here 
      strcpy(p) //error happen here, 
      // but You don’t know the source is the beginning of fun. 
   
      fun1(p); It will cause error in other place. 
   }
• Although when to use assert depends on context, Use assert to its fullest. precondition assertion to test the validity of the arguments passed to a method. and use postcondition assertion to test the validity of the results produced by the method. In my whirl2llvm project, I have used them a lot. It really gave me a lot of benefits.
  #include <cassert> 
  assert(I<5 && "I␣is␣more␣than␣5"); 
  //string literal is always true; 
  //when I<5 is false, the whole condition will be printed out.
• Assert is just if()+abort(): Difference of assert and return error(throw exception) lies in two sides: 1) It will abort you application, and you can use GDB to trace back source easily. 2)Do you think that is a bug or exceptions(error) ? Detail can be seen conclusion section.
• A practical example is dead battery in cell phone, It’s a exception. But if you have diarrhea, It’s a unnormal, It’s not exception. For example, opening file failure is exception, You should (throw exception). but age<0 is a indication of a bug.
  FILE *f = fopen("hr.dat".....); 
  if(f==nullptr){  // It’s an exception, so don’t use assert here. 
     return -1; 
     throw runtime_error(); 
  } 
   
  assert(age>=0); //Here use assert. 
  fprintf(f, %d, age);