Class C {
struct Something {
string s;
// Junk.
}
// map from some string to something.
map<string, Something> map;
// Some more code:
const Something *Lookup(string k) const {
const something *l = SomeLookUpFunction();
cout << l;
cout << &l->s;
cout << l->s;
return l;
}
}
// Some Test file
const C::Something *cs = C::Lookup("some_key");
cout << cs;
cout << &cs->s;
cout << cs->s;
The weird thing is this outputs:
* For lookup fucntion:
0x9999999
0x1277777
some_string
* For test code
0x9999999
0x1277777
000000000000000000000000000000000000000000 ....
In test file it gives a very long string of zeros, but the addresses are the same. Any idea what could be going wrong ?
Since you have not shared code for function SomeLookUpFunction, I have to guess that you are returning pointer to local object of type Something. This is a bad idea, see similar QA.
To start fixing your code, you should start by returning simple object, instead of pointer, as shown below:
// Some more code:
const Something lookup(string k) const {
const something l = SomeLookUpFunction(); // return simple object
cout << &l;
cout << &l.s;
cout << l.s;
return l; // same object
}
Of course you should improve the code by providing copy constructors for type something and even improving your map.
Related
It has been a while that I've written code in C/C++, and I've already found an alternative solution to my problem, but I would like to know why the original code doesn't work.
I have a test class which basically only stores a string.
class test {
private:
std::string name;
public:
test(std::string name) : name(name) {};
std::string get_name() { return name; }
};
In main I have a vector which I at one point fill with test objects. The code below simulates irregular usage of the vector vect.
int main(void) {
std::vector<test *> vect;
std::vector<test *>::iterator i;
//* Comment this for a working example
std::cout << "Searching empty vector" << std::endl;
i = *(_is_in_vector(vect, std::string("test 3")));
if (i == vect.end()) {
std::cout << "Nothing found" << std::endl;
} // */
vect.push_back(new test("test 1"));
vect.push_back(new test("test 2"));
vect.push_back(new test("test 3"));
std::cout << "All:" << std::endl;
i = *(_is_in_vector(vect, std::string("test 3")));
if (i != vect.end()) {
std::cout << "Erase " << (*i)->get_name() << std::endl;
vect.erase(i);
delete *i;
}
i = *(_is_in_vector(vect, std::string("test 3")));
if (i == vect.end()) {
std::cout << "Nothing found" << std::endl;
}
std::cout << "Left:" << std::endl;
for (i = vect.begin(); i!=vect.end(); ++i) {
std::cout << (*i)->get_name() << std::endl;
delete *i;
}
vect.clear();
return 0;
}
Because searching in the vector for a test object happens multiple times, I've created the function _is_in_vector that searches a test object and returns the iterator to it.
static std::vector<test *>::iterator * _is_in_vector(std::vector<test *> &vect, std::string find) {
std::string identity = find;
static std::vector<test *>::iterator i = vect.begin();
std::cout << "Vect size: " << vect.size() << std::endl;
for (i; i != vect.end(); ++i) {
std::string tmp = (*i)->get_name(); /* Segmentation fault after filling vector*/
if (0 == identity.compare(tmp)) break;
}
return &i;
}
The problem is that the code above works when I comment out the Searching empty vector part in main. Once the vector is filled with test objects, I call _is_in_vector a second time. The vector in this function does have three entries, but (*i) all point to NULL.
Output:
Searching empty vector
Vect size: 0
Nothing found
All:
Vect size: 3
Segmentation fault
Expected output:
Searching empty vector
Vect size: 0
Nothing found
All:
Vect size: 3
Erase test 3
Vect size: 2
Nothing found
Left:
test 1
test 2
First of all it is unclear why you need to store test objects by pointer but not by value. If you do need it use smart pointer.
As for your problem, why do you return pointer to an iterator? This is the root cause of your problem - to make &i legal to return you made it static, but static local variables initialized only once and do not change value btw calls - so after first call it pointed to an element in the vector, but after that you added elements and invalidated all iterators including static i hense the segmentation fault. So fix is simple - return iterator by value and make i non static but regular, it is light and it is totally fine to do so.
PS Identifiers starting with _ are illegal in global context, details can be found here What are the rules about using an underscore in a C++ identifier?
So your function actually should look like this:
static std::vector<test *>::iterator is_in_vector( std::vector<test *> &vect, const std::string &find)
{
return std::find_if( vect.begin(), vect.end(), [find]( test *p ) {
return p->get_name() == find;
} );
}
assuming the vector should never hold nullptr, if it is the case or to play safe change condition to:
return p && p->get_name() == find;
// In my Class A, I have many nodes and every node data is stored in a struct like this:
Class A
{
private:
struct BriteNodeInfo
{
int nodeId;
double xCoordinate;
double yCoordinate;
int inDegree;
int outDegree;
int asId;
std::string type;
};
};
// Each node instance is stored in a vector like this:
typedef std::vector<BriteNodeInfo> BriteNodeInfoList;
BriteNodeInfoList m_briteNodeInfoList;
//And then, here is the function that I want to implent down below
void SaveNodeData (std::string fname);
};
Problem: How do I implent that SaveNodeData() function to save my nodes data in .txt file like this?:
nodeId0 yCoordinate0 xCoordinate0
nodeId1 yCoordinate1 xCoordinate1
nodeId2 yCoordinate2 xCoordinate2
nodeId3 yCoordinate3 xCoordinate3
etc...
I have tried but my iteration syntax is not good enough. Here is my function, please help:
Here is my failed function:
void SaveNodeData (std::string fname)
{
ofstream os(fname.c_str(), ios::trunc);
vector<BriteNodeInfo> BriteNodeInfoList;
BriteNodeInfoList m_briteNodeInfoList;
for (BriteNodeInfoList::Iterator i = m_briteNodeInfoList.Begin(); i != m_briteNodeInfoList.End(); ++i)
{
os << BriteNodeInfo[i].nodeId "\t" << "\t" << BriteNodeInfo[i].yCoordinate; << "\t"BriteNodeInfo[i].xCoordinate<< "\n";
}
os << "\n";
}
Before starting, this code as written will clearly have some compile errors. But assuming you can manage to fix those issues, there's one huge flaw.
Your function SaveNodeData creates an empty BriteNodeInfoList, and then tries to read from it. The for loop you have written will always simply exit.
What you need to do is create and populate a BriteNodeInfoList somewhere that this function reads. You could pass it in as an argument to the function, have it as a private variable for class A (assuming SaveNodeData is made into a member of class A). Or you could make it a static member variable of class A (not really recommended: static member variables of objects have some serious problems).
void SaveNodeData (std::string fname) // file path
{
ofstream os(fname.c_str(), ios::trunc);
vector<BriteNodeInfo> BriteNodeInfoList = m_briteNodeInfoList;
for (std::vector<BriteNodeInfoList>::Iterator it = BriteNodeInfoList.Begin(); it != BriteNodeInfoList.End(); ++it)
{
os << (*it).nodeId << "\t" << (*it).yCoordinate << "\t" << (*it).xCoordinate << "\n";
}
}
I wrote a simple function to perform in place reversal:
void in_place_reverse(char *str){
if(!str || !(*str)){
return;
}
char *str_end = str+strlen(str)-1;
int temp;
while(str < str_end){
temp = *str;
*(str++) = *str_end;
*(str_end--) = temp;
}
}
I'm just wondering why when I do something like this:
char str[] = "Reverse me!";
cout << "Original: " << str << endl;
in_place_reverse(str);
cout << "Reversed: " << str << endl;
str wasn't changed inside of the function. The reason I ask is because the line *(str++) is incrementing the pointer that points to str. So what I'm really asking is why something like this isn't necessary:
char *str_beg = str;
char *str_end = str+strlen(str)-1;
int temp;
while(str_beg < str_end){
temp = *str_beg;
*(str_beg++) = *str_end;
*(str_end--) = temp;
}
So that we're not actually changing the pointer that points to the first position of str.
You actually are doing this implicitely because 'str' is passed by value (read: 'as a copy in a temporary variable').
To clarify this without the (distracting) pointer: consider
void increment(int x) {
x++;
}
int i = 1;
cout << i << endl;
increment(i);
cout << i << endl;
This will print '1' twice. The x that is seen inside the increment routine has the same value like the passed i. But it is not the same variable i. In fact it is a copy of i. When we return from the routine, the copy is discarded. Further reading: This would be different if we'd pass x by reference, like so:
void increment(int &x) {
x++;
}
The declaration of the function void in_place_reverse(char *str) results in a copy of the pointer being created when the function is called, in a variable called str that is private and local to the in_place_reverse. You can modify this value all you like without affecting the original that exists in the scope of the calling function.
I just came from Java and Python world to C++ world, and faced a problem while trying to get the value from a public const function of a class.
I have a class as follows:
class CMDPoint
{
public:
CMDPoint();
CMDPoint(int nDimensions);
virtual ~CMDPoint();
private:
int m_nDimensions; // the number of dimensions of a point
float* m_coordinate; // the coordinate of a point
public:
const int GetNDimensions() const { return m_nDimensions; }
const float GetCoordinate(int nth) const { return m_coordinate[nth]; }
void SetCoordinate(int nth, float value) { m_coordinate[nth] = value; }
};
Ultimately, I wish to write all the clusterPoints in clusterPointArray into the file. However, now I am just testing it with the first clusterPoint (thus, GetCoordinate(0)).
ofstream outFile;
outFile.open("C:\\data\\test.txt", std::ofstream::out | std::ofstream::app);
for (std::vector<CMDPoint> ::iterator it = clusterEntry->clusterPointArray.begin(); it != clusterEntry->clusterPointArray.end(); ++it)
{
outFile << ("%f", (*it).GetCoordinate(0)); // fails
outFile << " ";
}
outFile << "\n";
outFile.close();
The problem is I only see the " " in the file. No coordinate has been written in. Did I do anything wrong while fetching the value from const float GetCoordinate(int nth)?
try to change this
outFile << ("%f", (*it).GetCoordinate(0)); // fails
to this:
outFile << (*it).GetCoordinate(0); // OK
Because the ("%f", (*it).GetCoordinate(0)) represents nothing , only a enumerations of expressions separated by , . It will not be evaluated into a pair of objects as in java is i think.
Edit:("%f", (*it).GetCoordinate(0)) actually evaluates to the last element that is (*it).GetCoordinate(0) ( PlasmaHH comment ) so it should still print something. However if nothing is printed then the collection clusterEntry->clusterPointArray could be empty and the code inside the for loop might not be executed ever.
Hope this helps,
Razvan.
outFile << it->GetCoordinate(0);
I have the following functor:
class ComparatorClass {
public:
bool operator () (SimulatedDiskFile * file_1, SimulatedDiskFile * file_2) {
string file_1_name = file_1->getFileName();
string file_2_name = file_2->getFileName();
cout << file_1_name << " and " << file_2_name << ": ";
if (file_1_name < file_2_name) {
cout << "true" << endl;
return true;
}
else {
cout << "false" << endl;
return false;
}
}
};
It is supposed to be a strict weak ordering, and it's this long (could be one line only) for debug purposes.
I'm using this functor as a comparator functor for a stl::set. Problem being, it only inserts the first element. By adding console output to the comparator function, I learned that it's actually comparing the file name to itself every time.
Other relevant lines are:
typedef set<SimulatedDiskFile *, ComparatorClass> FileSet;
and
// (FileSet files_;) <- SimulatedDisk private class member
void SimulatedDisk::addFile(SimulatedDiskFile * file) {
files_.insert(file);
positions_calculated_ = false;
}
EDIT: the code that calls .addFile() is:
current_request = all_requests.begin();
while (current_request != all_requests.end()) {
SimulatedDiskFile temp_file(current_request->getFileName(), current_request->getResponseSize());
disk.addFile(&temp_file);
current_request++;
}
Where all_requests is a list, and class Request is such that:
class Request {
private:
string file_name_;
int response_code_;
int response_size_;
public:
void setFileName(string file_name);
string getFileName();
void setResponseCode(int response_code);
int getResponseCode();
void setResponseSize(int response_size);
int getResponseSize();
};
I wish I could offer my hypotesis as to what's going on, but I actually have no idea. Thanks in advance for any pointers.
There's nothing wrong with the code you've posted, functionally speaking. Here's a complete test program - I've only filled in the blanks, not changing your code at all.
#include <iostream>
#include <string>
#include <set>
using namespace std;
class SimulatedDiskFile
{
public:
string getFileName() { return name; }
SimulatedDiskFile(const string &n)
: name(n) { }
string name;
};
class ComparatorClass {
public:
bool operator () (SimulatedDiskFile * file_1, SimulatedDiskFile * file_2) {
string file_1_name = file_1->getFileName();
string file_2_name = file_2->getFileName();
cout << file_1_name << " and " << file_2_name << ": ";
if (file_1_name < file_2_name) {
cout << "true" << endl;
return true;
}
else {
cout << "false" << endl;
return false;
}
}
};
typedef set<SimulatedDiskFile *, ComparatorClass> FileSet;
int main()
{
FileSet files;
files.insert(new SimulatedDiskFile("a"));
files.insert(new SimulatedDiskFile("z"));
files.insert(new SimulatedDiskFile("m"));
FileSet::iterator f;
for (f = files.begin(); f != files.end(); f++)
cout << (*f)->name << std::endl;
return 0;
}
I get this output:
z and a: false
a and z: true
z and a: false
m and a: false
m and z: true
z and m: false
a and m: true
m and a: false
a
m
z
Note that the set ends up with all three things stored in it, and your comparison logging shows sensible behaviour.
Edit:
Your bug is in these line:
SimulatedDiskFile temp_file(current_request->getFileName(), current_request->getResponseSize());
disk.addFile(&temp_file);
You're taking the address of a local object. Each time around the loop that object is destroyed and the next object is allocated into exactly the same space. So only the final object still exists at the end of the loop and you've added multiple pointers to that same object. Outside the loop, all bets are off because now none of the objects exist.
Either allocate each SimulatedDiskFile with new (like in my test, but then you'll have to figure out when to delete them), or else don't use pointers at all (far easier if it fits the constraints of your problem).
And here is the problem:
SimulatedDiskFile temp_file(current_request->getFileName(),
current_request->getResponseSize());
disk.addFile(&temp_file);
You are adding a pointer to a variable which is immediately destroyed. You need to dynamically create your SDF objects.
urrent_request = all_requests.begin();
while (current_request != all_requests.end()) {
SimulatedDiskFile temp_file(...blah..blah..); ====> pointer to local variable is inserted
disk.addFile(&temp_file);
current_request++;
}
temp_file would go out of scope the moment next iteration in while loop. You need to change the insert code. Create SimulatedDiskFile objects on heap and push otherwise if the objects are smaller then store by value in set.
Agree with #Earwicker. All looks good. Have you had a look inside all_requests? Maybe all the filenames are the same in there and everything else is working fine? (just thinking out loud here)