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Using the following code:
class MyClass
{
public:
void GetStrings(list<std::string>& strs)
{
strs = m_strings;
}
list<std::string> m_strings;
}
Are all m_strings's elements being copied when strs = m_strings; is performed?
In other words:
Is this equal to list<std::string>& strs = m_strings; or to list<std::string> strs = m_strings; (with respect to elements copy)
yes.
Now, if you wanted to copy just a reference to the list (like you have in languages such as C# or Java) then pass a shared_ptr instead, then you will have a single list with 2 references to it.
All variable assignment in C/C++ is done by value, even if the variable being passed is a pointer (eg a 4 byte variable). Other languages are the same, its just that they cover this up with their language constructs.
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Consider the following source code:
map<string,vector<SectionElement *>> _sections;
...
static SharedData *_shared;
...
static int iniHandler(void* user, const char* section, const char* name,
const char* value)
{
map<string,vector<SectionElement*>> iniFile = *(_shared->sections);
auto& iniSection = iniFile[ section];
auto sectionElement = new SectionElement();
sectionElement->name = name;
sectionElement->value = value;
iniSection.push_back( sectionElement);
return 1;
}
The problem with the code is that if I add an element to iniSection it works, but the vector that is retrieved from iniFile does not seem to be the same that is kept in the map. So every time the function iniHandler is called the count of the vector is zero. I am a bit at a loss here and wondering what obvious thing I am missing...
You modify a local map called iniFile. This has no effect on some other map, *(_shared->sections). Perhaps you wanted to make iniFile a reference?
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How do I correctly ask for object rock in main. I simplified the code, figuring this was the only problem. The error "expected primary-expression before '&' token.
void createObject(vector <object>& obj, world wld)
{
....
}
int main()
{
object rock;
createObject(vector<object>& rock, level_1);
return 0;
}
Very simply:
int main()
{
std::vector<object> rock_vector(1);
createObject(rock_vector, level_1);
}
You can't pass rock to it, as it's not a vector. You need to pass an actual vector to it. Here, I made rock_vector of size 1, so it's at least got one object in it (so rock_vector[0] is more or less your replacement for rock).
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I'd like to write a function that will return 4 members of a class. They are 3 ints and char, and I'd like to store them all in one vector and return it from a function call. Can I do that?
You either need an std::tuple if you want to preserve the types and if the result length is constant, or just cast all members to some common supertype and store them in a container.
You need a class:
struct S
{
int a, b, c;
char letter;
};
int main()
{
S s;
}
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If I declare a function:
vector<int> & call(string *target)
How should I use target in the function for a comparison with another string? For example:
string str;
if(str == //string in target){
...
}
As &target, or simply target? Also, how should the return look? I'm assuming it should be:
return &some_vector;
since that is the type in the function declaration. Finally, what about the opposite? That is:
vector<int> & call(string &target)
When in the function and wanting to use the string, is it as simple as:
*target
On your first question it's
if(str == *target){
On your second question my advice would simply be, don't. It looks like you are trying to return a reference to a local variable. That is a well known newbie mistake that will simply crash your program.
On your third question it's
if(str == target){
You need to dereference the pointer to get the object. That is, use *:
if (str == *target) {
Also, how should the return look?
You're returning a reference, not a pointer. That means you simply need to return the name of the object:
return some_vector;
Finally, what about the opposite?
If target is a reference (NOT a pointer) than you simply use the name of the object. There's no dereference involved with this.
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For example:
string str[3];
void foo(char** str)
{
//do something to str...
}
How to pass str[] to function foo in a convenient way?
The function expects an array of pointers, so you'll have to make one from your array of strings:
std::vector<char*> pointers;
for (auto & s : str) {
pointers.push_back(&s[0]);
}
foo(&pointers[0]);
Beware that this may not be valid if the function modifies the pointers, or the strings they point to. A better option would be to avoid mixing C and C++ style string handling, if possible.