6-30 - pagination in CPPCMS, The end of June

Task: Use only C++ to read logs in Mongodb and do the pagination according to the page_size , page_index, order_column_name, is_ascending.


Problem encountered: 
1. Read from Mongodb, sort by order_column_name, skip to the place where pagination needed, etc.
2. Parameters like page_size as -1 and page_index as -1 should be handled by throw, try, catch, which I did not know what to do in the first.
3. Segmentation fault caused by an char[] without dictating the size of it.
4. The design of storing errors. Still, I think error with shared_ptr would be more fit into this case. But anyway, I did it with Errors.


Problem solved:
1. Internet, Mongodb's own site of specifying functions. Tried many times.
2. Internet, I did make a silly mistake before---let a class named ErrorPage inherit from exception. But still, I understand myself---I just want to use the functions of exception, it means ErrorPage has exceptions, which is of course not right.
Build a class named ErrorPageExceptions, override the function  "virtual const char* what() const noexcept", and this is done.
3. Have a hard time fixing it. Knowing this is caused by visiting or writing internal storage and knowing which sentence " throw error_page_exceptions("too much error specified!");"caused this, but I didn't know how to fix it.

first, I thought it's because lifetime problem, cause I init a variable by the construction exception. So I write SetErrorName to rewrite the char* error.

But it's not this mistake. 

Then I thought maybe it's because I haven't initialize char*.

Not this mistake again.

I finally give size to char*, and it's all right. 

Pointer is a dangerous thing.

Afterwards, it turns out that use invalid_argument is all for the try, throw, catch exception.

内置类型普通传参比传引用快。