How do I initialize a table inside a class definition?

Question

I am building a parser using C++ 11/14 (Visual Studio 2015). How do I create and initialize a table within the class, and have it compute the length of the table?

I get the following errors from the compiler:

1>d:\temp\win32project1\win32project1\source1.cpp(49): warning C4200: nonstandard extension used: zero-sized array in struct/union

1> d:\temp\win32project1\win32project1\source1.cpp(49): note: This member will be ignored by a defaulted constructor or copy/move assignment operator

1>d:\temp\win32project1\win32project1\source1.cpp(49): error C2997: 'tlv::_requests': array bound cannot be deduced from an in-class initializer

1> d:\temp\win32project1\win32project1\source1.cpp(45): note: see declaration of 'tlv::_requests'

typedef enum _TLV_TYPE : UINT32
    {
    tlv_type_read,
    tlv_type_write,
    tlv_type_param,
    tlv_type_status
    } TLV_TYPE;

typedef struct
    {
    TLV_TYPE    type;
    UINT32      length;
    } TLV_RECORD, *pTLV_RECORD;

typedef std::function <UINT32 (pTLV_RECORD Record)> pTLV_PARSER;

typedef struct
    {
    TLV_TYPE            type;
    pTLV_PARSER         parse_routine;
    vector <TLV_RECORD> parameters;
    vector <TLV_RECORD> response;
    } TABLE, *pTABLE;

class tlv
{
public:
    tlv (SOCKET Socket);
    ~tlv ();
    UINT32 start_parse (pTLV_RECORD Record);

protected:
    pTLV_PARSER parse_read;
    pTLV_PARSER parse_write;

const TABLE _requests [] =
    {
        {tlv_type_read, parse_read, {{tlv_type_param, 4}, {tlv_type_param, 0}, {tlv_type_param, 4}}, {{tlv_type_status, 4}}},
        {tlv_type_write, parse_write,{{tlv_type_param, 4}}, {{tlv_type_status, 4}}},
    };

};  // End of class tlv

Possible duplicate of [C++ typedef member function signature syntax](http://stackoverflow.com/questions/4832275/c-typedef-member-function-signature-syntax) — Barmar, Apr 14 '17 at 21:17
`typedef struct/enum _foo { ... } foo;` is an anti-pattern in C++. The proper way to write your definition would be `enum TLV_TYPE : UINT32 { ... };`. The `typedef` crap you're doing is used in C, but not C++. — cdhowie, Apr 14 '17 at 21:33
This code [compiles fine for me](http://coliru.stacked-crooked.com/a/fbd3ee2eda41d9c7). If you are getting errors, please include them in your question. — cdhowie, Apr 14 '17 at 21:36
@cdhowie, I am getting these errors: warning C4200: nonstandard extension used: zero-sized array in struct/union note: This member will be ignored by a defaulted constructor or copy/move assignment operator error C2997: 'tlv_::_requests': array bound cannot be deduced from an in-class initializer note: see declaration of 'tlv_::_requests' error C2229: class 'tlv_' has an illegal zero-sized array — Brian, Apr 14 '17 at 23:44
@Brian What happens if you declare the length of the array? `const TABLE _requests[2] = { ... };` (See [this answer](http://stackoverflow.com/a/29593683/501250) which explains why, according to the standard, object member array lengths cannot be deduced by their initializers. Note that, while G++ accepts the code without a diagnostic, Clang rejects it with the same error you received.) — cdhowie, Apr 15 '17 at 02:53
@cdhowie, Yes, adding the length of the array works. Thank you for your help! — Brian, Apr 15 '17 at 17:25

score 0 · Answer 1 · answered Apr 14 '17 at 21:28

0

The type to store a pointer to a member function is declared like that:

typedef int (tlv::*pTLV_PARSER)(pTLV_RECORD Record);

I'm not clear on what exactly your parser functions return, so just used an int data type in declaration

answered Apr 14 '17 at 21:28

YePhIcK

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score 0 · Accepted Answer · answered Apr 15 '17 at 17:32

For the reasons specified in the link provided by @cdhowie in the comments, the array length must be specified because the C++ standard, for reasons best known to the committee, has rather bizarre and non-obvious rules on how member initializers are treated (see the link, and the link in the link), and the interpretation of these rules vary from one compiler to another.

How do I initialize a table inside a class definition?

2 Answers2