In gdb, I can call some class functions, but others "cannot be resolved". Why?

Question

I have not worked on shared pointers yet .. I just know the concept. I'm trying to debug functions in the following c++ class, which stores data of an XML file (read-in via the xerces library).

// header file
class ParamNode;
typedef boost::shared_ptr<ParamNode> PtrParamNode;

class ParamNode : public boost::enable_shared_from_this<ParamNode> {
public:
   ...
   typedef enum { DEFAULT, EX, PASS, INSERT, APPEND } ActionType;
   bool hasChildren() const;
   PtrParamNode GetChildren();
   PtrParamNode Get(const std::string&  name, ActionType = DEFAULT );
protected:
   ....
   ActionType defaultAction_;
}

Now if I'm debugging a piece of code in which I have an instance of the pointer to the class ParamNode, and it's called paramNode_

PtrParamNode paramNode_;
// read xml file with xerces
paramNode_ = xerces->CreateParamNodeInstance();
// now, the xml data is stored in paramNode_.
std::string probGeo;
// this works in the code, but not in (gdb)!!
paramNode_->Get("domain")->GetValue("gt",probGeo);
cout << probGeo << endl; // <-- breakpoint HERE

Using gdb I'm inspecting the paramNode_ object:

(gdb) p paramNode_
$29 = {px = 0x295df70, pn = {pi_ = 0x2957ac0}}
(gdb) p *paramNode_.px
$30 = {
  <boost::enable_shared_from_this<mainclass::ParamNode>> = {weak_this_ = {px = 0x295df70, pn = {pi_ = 0x2957ac0}}}, 
  _vptr.ParamNode = 0x20d5ad0 <vtable for mainclass::ParamNode+16>, 
  ...
  name_= {...}, 
  children_ = {size_ = 6, capacity_ = 8, data_ = 0x2969798},
  defaultAction_ = mainclass::ParamNode::EX, }

and print its members:

(gdb) ptype *paramNode_.px
type = class mainclass::ParamNode : public boost::enable_shared_from_this<mainclass::ParamNode> {
  protected:
    ...
    mainclass::ParamNode::ActionType defaultAction_;
  public:
    bool HasChildren(void) const;
    mainclass::PtrParamNode GetChildren(void);
    mainclass::PtrParamNode Get(const std::string &, mainclass::ParamNode::ActionType);

However, I can only call the functions HasChildren or GetChildren, whereas calling Get from gdb results in an error:

(gdb) p (paramNode_.px)->HasChildren()
 $7 = true
 (gdb) p (paramNode_.px)->GetChildren()
 $8 = (mainclass::ParamNodeList &) @0x295dfb8: {
   size_ = 6, 
   capacity_ = 8, 
   data_ = 0x29697a8
  }
(gdb) p (paramNode_.px)->Get("domain")
 Cannot resolve method mainclass::ParamNode::Get to any overloaded instance
(gdb) set overload-resolution off
(gdb) p (paramNode_.px)->Get("domain")
 One of the arguments you tried to pass to Get could not be converted to what the function wants.
(gdb) p (paramNode_.px)->Get("domain", (paramNode_.px).defaultAction_)
 One of the arguments you tried to pass to Get could not be converted to what the function wants.

In the code, executing the Get("domain") function works just fine. Why is that? I'm thankful if you include explanations in your answer, due to my limited knowledge of shared pointers.

Solution 1

gdb is not a compiler, it will not do the (not-so-)nice user-defined type conversions for you. If you wish to call a function that wants a string, you need to give it a string, not a const char*.

Unfortunately, gdb cannot construct an std::string for you on the command line, again because it is not a compiler and object creation is not a simple function call.

So you will have to add a little helper function to your program, that would take a const char* and return an std::string&. Note the reference here. It cannot return by value, because then gdb will not be able to pass the result by const reference (it's not a compiler!) You can choose to return a reference to a static object, or to an object allocated on the heap. In the latter case it will leak memory, but this is not a big deal since the function is meant to be called only from the debugger anyway.

std::string& SSS (const char* s)
{
    return *(new std::string(s));
}

Then in gdb

gdb> p (paramNode_.px)->Get(SSS("domain"))

should work.

Solution 2

In such a situation I just had success after giving the command

set overload-resolution off

Solution 3

A couple additions to the previous answer --

gdb will probably eventually learn how to do conversions like this. It can't now; but there is active work on improving support for C++ expression parsing.

gdb doesn't understand default arguments, either. This is partly a bug in gdb; but also partly a bug in g++, which doesn't emit them into the DWARF. I think DWARF doesn't even define a way to emit non-trivial default arguments.

Solution 4

n.m.'s answer is great, but to avoid having to edit your code and recompile take a look at the solution given in Creating C++ string in GDB.

In that solution they demonstrate allocating space for a std::string on the heap and then initializing a std::string to pass into the function they'd like to call from gdb.

View more solutions

Comments

[Sebastian]

yes, this looks good sir. I need to supply all arguments though (the DEFAULT second parameter needs to be supplied as well). (gdb) p (paramNode_.px)->Get(SSS("domain")) Too few arguments in function call.. However, this works: (gdb) p (paramNode_.px)->Get(SSS("domain"), (paramNode_.px).defaultAction_).px $28 = (mainclass::ParamNode *) 0x2969710. Thanks for the detailed comments!

[Sebastian]

thanks for this addition. Are you aware of any debuggers which can do this already?

[mallwright]

It would be great if this answer could be expanded a bit further. How does setting this parameter solve the problem?

[ealfonso]

in 2018 it seems that it still doesn't know how to do it. at least in the version I am using: 7.9-gg19

[John H.]

See also stackoverflow.com/questions/42462860/… :-)

[Lekensteyn]

set overload-resolution off "Disable overload resolution for C++ expression evaluation. For overloaded functions that are not class member functions, GDB chooses the first function of the specified name that it finds in the symbol table, whether or not its arguments are of the correct type. For overloaded functions that are class member functions, GDB searches for a function whose signature exactly matches the argument types." sourceware.org/gdb/current/onlinedocs/gdb/…

[JohnK]

Wait, just want to get this straight: is gdb not a compiler?

[user202729]

Actually it's possible Creating C++ string in GDB - Stack Overflow