False positive: error : Using object that is a temporary.

[Jean Porcherot]

This piece of code generates "error : Using object that is a temporary.".

#include <string>
#include <vector>
#include <sstream>

inline void MyStrCp( char* Dst, size_t DstSize, const char* Src )
{
#ifdef WIN32
    strcpy_s(Dst, DstSize, Src);
#else
    strcpy(Dst, Src);
#endif
}

class MyChannelInfo
{
public:
    MyChannelInfo()
    {
    }
    ~MyChannelInfo()
    {
        delete m_name;
    }

    MyChannelInfo( const char* name )
    {
        m_name = new char[strlen(name)+1];
        MyStrCp( m_name, strlen(name)+1, name );
    }

    MyChannelInfo& operator=( const MyChannelInfo& info )
    {
        if ( this != &info )
        {
            delete m_name;
            m_name = new char[strlen(info.m_name)+1];
            MyStrCp( m_name, strlen(info.m_name)+1, info.m_name );
        }

        return *this;
    }
    MyChannelInfo( const MyChannelInfo& info )
    {
        operator=( info );
    }

    inline const char* GetName() const { return m_name; }

    char* m_name = NULL;
};

typedef std::vector<MyChannelInfo> MyChannelInfos;

class MyChannel
{
public:
    MyChannel()
    {
    }

    std::string GetComment()
    {
        std::stringstream str;
        str << "My comment";
        return str.str();
    }
};

class FillVector
{
    MyChannelInfos& channels;

public:

    FillVector( MyChannelInfos& _channels ) : 
        channels( _channels )
    {}
    virtual void Visit1( MyChannel& channel ) 
    { 
        channels.push_back( MyChannelInfo( channel.GetComment().c_str() ) );
    }

    virtual void Visit2()
    {
        channels.push_back( MyChannelInfo( "foo" ) ); // reports error : Using object that is a temporary.
    }
};

The tool seems lost, because it reports this error in Visit2, while it looks like Visit1 is actually causing problem: If I replace MyChannel::GetComment by:

const std::string& GetComment()
    {
        static std::string comment = "";
        return comment;
    }

Then the problem disappear...

However, retruning a std::string by copy in GetComment is perfectly OK (In my original code, the GetComment returns a string computed from other attributes of the class, no way to retrun a const reference). The string remains valid while MyChannelInfo constructor is called and char* copied to MyChannelInfo::m_name.

There should be no error here.

[Oleksandr Labetskyi]

Greetings Jean. Would you be so kind to provide information about which cppcheck version was used with this example?

[Daniel Marjamäki]

Thanks! I have created this ticket: https://trac.cppcheck.net/ticket/13244