The full warning message looks like this:
warning: ‘XXX’ has no out-of-line virtual method definitions; its vtable
will be emitted in every translation unit [-Wweak-vtables]
This warning usually comes from code like:
// XXX.h中
class BaseData
{
public:
virtual ~BaseData() = default;
int _base_data = 0;
};
class DerivedDataA final : public BaseData
{
public:
int _derived_data_a = 0;
};
class DerivedDataB final : public BaseData
{
public:
int _derived_data_b = 0;
};
In this header, a base class BaseData and two derived classes DerivedDataA and DerivedDataB are declared. The idea was to describe two kinds of data classes A and B with some commonality. If that was all, polymorphism would not be needed, and the base destructor could be non‑virtual. But since the use case required storing both derived types in a common container like std::vector<BaseData*>, the base destructor had to be virtual. With RTTI or dynamic_cast, type distinction is possible. At this point, including the header in multiple cpp files triggers the warning:
warning: ‘BaseData’ has no out-of-line virtual method definitions; its vtable
will be emitted in every translation unit [-Wweak-vtables]
In fact, all three classes receive the same warning. The warning does not affect runtime behavior, but it means each translation unit generates a copy of the vtable, later deduplicated at link time. This slows linking. To avoid it, provide an out‑of‑line definition for at least one virtual function. For example, defining a destructor body works, but that makes the class non‑trivial. If cleanup is needed, that is fine, but in this example no special cleanup was required. So instead a harmless virtual function was added:
// XXX.h
class BaseData
{
public:
virtual ~BaseData() = default;
virtual std::string getType();
int _base_data = 0;
};
class DerivedDataA final : public BaseData
{
public:
std::string getType() override;
int _derived_data_a = 0;
};
class DerivedDataB final : public BaseData
{
public:
std::string getType() override;
int _derived_data_b = 0;
};
// XXX.cpp
#include "XXX.h"
std::string BaseData::getType()
{
return "base";
}
std::string DerivedDataA::getType()
{
return "derived_a";
}
std::string DerivedDataB::getType()
{
return "derived_b";
}
This removes the warning and reduces link overhead.
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