c++ "no matching function for call to" error with structure

Question

I have C++ code that maps GUID(unsigned long) to structure.

#include <string>
#include <map>
#include <iostream>

typedef unsigned long GUID;

enum Function {
  ADDER = 1,
  SUBTRACTOR = 2,
  MULTIPLIER = 3,
  SQUAREROOT = 4
};

struct PluginInfo
{
    GUID guid;
    std::string name;
    Function function;

    PluginInfo(GUID _guid, std::string _name, Function _function) {guid = _guid, name = _name, function = _function;}
};

typedef std::map<GUID, PluginInfo> PluginDB;

PluginInfo temp1(1, "Adder", ADDER);
PluginInfo temp2(2, "Multiplier", MULTIPLIER);

PluginDB::value_type pluginDbArray[] = {
    PluginDB::value_type(1, temp1),
    PluginDB::value_type(2, temp2)
};

const int numElems = sizeof pluginDbArray / sizeof pluginDbArray[0];
PluginDB pluginDB(pluginDbArray, pluginDbArray + numElems);

int main()
{
    std::cout << pluginDB[1].name << std::endl;
}

When I compile it, I got error message.

/usr/include/c++/4.2.1/bits/stl_map.h: In member function ‘_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = long unsigned int, _Tp = PluginInfo, _Compare = std::less, _Alloc = std::allocator >]’: mockup_api.cpp:58: instantiated from here /usr/include/c++/4.2.1/bits/stl_map.h:350: error: no matching function for call to ‘PluginInfo::PluginInfo()’ mockup_api.cpp:29: note: candidates are: PluginInfo::PluginInfo(GUID, std::string, Function) mockup_api.cpp:24: note:
PluginInfo::PluginInfo(const PluginInfo&)

What might be wrong?

Possible duplicate of Cannot access struct value in std::map — Yakk - Adam Nevraumont
– Yakk - Adam Nevraumont, Commented Apr 15, 2016 at 1:50

Jason · Accepted Answer · 2011-04-18 20:56:33Z

3

Any objects you place in a STL container initialized with an initial number of objects (i.e., you're not initializing an empty container) must have at least one default constructor ... yours does not. In other words your current constructor needs to be initialized with specific objects. There must be one default constructor that is like:

PluginInfo();

Requiring no initializers. Alternatively, they can be default initializers like:

PluginInfo(GUID _guid = GUID(), 
           std::string _name = std::string(), 
           Function _function = Function()): 
           guid(_guid), name(_name), function(_function) {}

edited Apr 18, 2011 at 20:56

answered Apr 18, 2011 at 20:32

Jason

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7 Comments

user2100815 Over a year ago

If you mean that they must have a default constructor, this is wrong. STL containers can certainly contain objects that do not have default constructors. The requirement is that they be copyable and assignable.

Jason Over a year ago

No, sorry, didn't explicitly mean that ... I've tried to modify my answer to correct that assumption and show you can have alternate constructors, but they must be something that can be initialized without having to pass in any explicit parameters by the user.

user2100815 Over a year ago

@Jason Such a constructor is a default constructor - and your answer is still wrong.

Jason Over a year ago

So you can create a constructor for an object that requires explicit user-passed parameters (no default initializers), and store that object in an STL container? Isn't that the error that the compiler is throwing?

user2100815 Over a year ago

@Jason Yes, otherwise storing things like BankAccount (which can have no sensible defaults) in STL containers would not be possible. As to what the problem with the OP's code is - I couldn't say, as soon as I see a use of sizeof in C++ code my brain turns off and I feel no need to turn it back on again in this case.

|

user2100815 · Accepted Answer · 2011-04-18 21:15:41Z

3

The problem is that when you say:

 pluginDB[1]

you try to create an entry in the map (because [1] does not exist) and to do that as Jason points out, you need a default constructor. However, this is NOT a general requirement of standard library containers, only of std::map, and only of operator[] for std::map (and multimap etc.), which is a good reason why IMHO operator[] for maps et al should be done away with - it is far too confusing for new C++ programmers, and useless for experienced ones.

answered Apr 18, 2011 at 21:15

user2100815

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