c++ - Initialize private array with unknown number of elements (C++98) -

looks missed fundamental here, haven't worked out.

below snippet , corresponding output.

what wanted is: - declare , initialize array of structs, without knowing number of elements in advance. - ideally array , number of elements private members.

what tried:

• declared m_member_tab[] , m_num_members private.
• created init() function initializes m_member_tab[] , calculate m_num_members.

outcome:

• m_member_tab[] initialized ok (see below output).

• but inside constructor (after calling init), m_member_tab corrupted.

  #include <iostream> using std::cout; using std::endl;  class tarraytest {     public:         tarraytest();      private:         void init();          typedef struct _tmember         {             int m_key;             int m_val;         }         tmember;          tmember m_member_tab[];         int m_num_members; };  tarraytest::tarraytest() {     init();     cout << "ctor: number of elements = " << m_num_members << endl;     for( int = 0; < m_num_members; i++ )     {         cout << "ctor: "             << "key " << m_member_tab[i].m_key             << " - val " << m_member_tab[i].m_val             << endl;     } };  void tarraytest::init() {     tmember m_member_tab[] =     {         { 1, 100 },         { 2, 200 },         { 3, 300 },         { 4, 400 },         { 5, 500 },     };     m_num_members = sizeof( m_member_tab ) / sizeof( tmember );     cout << "init: number of elements = " << m_num_members << endl;     for( int = 0; < m_num_members; i++ )     {         cout << "init: "             << "key " << m_member_tab[i].m_key             << " - val " << m_member_tab[i].m_val             << endl;     } }  int main() {     tarraytest test; } 

output:

    init: number of elements = 5     init: key 1 - val 100     init: key 2 - val 200     init: key 3 - val 300     init: key 4 - val 400     init: key 5 - val 500     ctor: number of elements = 5     ctor: key 5 - val 32766     ctor: key 0 - val 0     ctor: key 0 - val 0     ctor: key -1212526907 - val 32623     ctor: key 0 - val 0 

this member declaration:

    tmember m_member_tab[]; 

is not valid c++.

there additional problem in init function, declare local variable of same name, doesn't matter: above declaration not invalid, since it's not @ end of struct doesn't make sense language extension.

instead, use std::vector, this:

    std::vector<tmember> m_member_tab; 

it keeps track of array size, don't need member.

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in other news, c++ directly supports initialization of instance of class. should not define ordinary function that. instead use language mechanism that, namely constructor.

you can find information constructors in tutorial , introductory c++ textbook.

the language supported mechanism has many advantages compared init function.

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oh, , seeing each item in array contain key , value, consider std::map, or, if can use c++11 , not c++98/c03, std::unordered_map (faster no sorted traversal of keys).