Question

I have a simple class

    class symbol_entry
{
private:
    static unsigned long uid;

public:
    std::string name;
    std::string filename;
    unsigned int line_number;
    unsigned int column_number;
    symbol_entry* parent_symbol;
    std::map<const char*,symbol_entry*> child_symbols;
    unsigned long type_flags;

public:
    symbol_entry();
    symbol_entry(const char* name,
                 const char* filename,
                 int line_number,
                 int column_number,
                 symbol_entry* parent_symbol,
                 unsigned long type_flags);
    ~symbol_entry();

    symbol_entry* get_child(const char* name);
    bool put_child(symbol_entry* child);
};

here is the implementation of symbol_entry::put_child;

bool symbol_entry::put_child(symbol_entry* child)
{   
    if(child_symbols[child->name.c_str()])
        return false;
    child_symbols.insert(std::make_pair(child->name.c_str(),child));
    return true;
}

whenever i perform a test like this;

symbol_entry* tsym=new symbol_entry("test","$",0,0,0,0);
symbol_entry* tcsym=new symbol_entry("test_child","$",0,0,0,0);
tsym->put_child(tcsym);
std::cout<<tsym->child_symbols.begin()->first<<" => "<<tsym->child_symbols.begin()->second<<std::endl;

child_symbols.begin()->second is storing a null pointer. I cannot work this out and have tried many variations including const and references with to avail.

Answer 1

child_symbols[child->name.c_str()] will always create and return a new map entry (a NULL one), and then child_symbols.insert(...) doesn’t do anything (so the value in the map stays NULL). The correct way to check whether the key is already in the map is to use find:

if (child_symbols.find(...) != child_symbols.end()) // already exists