It's easier to explain with some code:
#include <iostream>
using namespace std;
struct test {
int one;
int two;
};
void insertdata(test & info)
{
cin >> info.one;
cin >> info.two;
}
int doitnow(test mytable[])
{
test info;
int i = 0;
for (i=0; i<3; i++) {
insertdata(test & info);
mytable[i] = info;
}
return i;
}
int main()
{
int total;
test mytable[10];
total = doitnow(test mytable[]);
cout << total;
}
So, I need to pass info by reference to the function insertdata, I need to use that function in doitnow to fill up a table and I need to show in the main function the number of items inserted in doitnow. I keep getting errors when I try to call functions:
teste.cpp: In function ‘int doitnow(test*)’:
teste.cpp:21:29: error: expected primary-expression before ‘&’ token
insertdata(test & info);
teste.cpp: In function ‘int main()’:
teste.cpp:33:30: error: expected primary-expression before ‘mytable’
total = doitnow(test mytable[]);
So, probably it's an obvious mistake but I'm a beginner at this.
Thanks for your help.
test& info is a definition. If you pass something to a function, you write an expression like info. It is automatically passed by reference because you specified info as a reference in the formal parameter list.
You also wrote
mytable[i] = info;
and not
mytable[i] = test & info;
didn't ya?
Use insertdata(info); instead.
Same goes for arrays. Use doitnow(test) instead.
Related
I am facing an issue hashing __uint128_t. Following is my code:
#include <iostream>
int main () {
__uint128_t var = 1;
std::cout << std::hash<__uint128_t> () (var) << "\n";
return 0;
}
I am getting the error as:
test.cpp: In function ‘int main()’:
test.cpp:5:40: error: use of deleted function ‘std::hash<__int128 unsigned>::hash()’
5 | size_t h = std::hash<__uint128_t> () (var);
| ^
How can I get the hash for __uint128_t? (Probably a very basic question but I have been stuck here for a while). Also, I would like to know the meaning of the error. Thanks in advance.
Taking a look at the docs on https://en.cppreference.com/w/cpp/utility/hash. You will have to write your own.
Here is some code for what a basic __uint128_t hash function might look like:
namespace std {
template<>
struct hash<__uint128_t> {
size_t operator()(__uint128_t var) const {
return std::hash<uint64_t>{}((uint64_t)var ^ (uint64_t)(var >> 64));
}
};
}
Note not tested or compiled.
I'm trying to make a programming that would set the time to 11/7/2018, but it's not working. It only display 4 errors. Can someone please help me rectify the code.
#include <iostream>
using namespace std;
class date {
private:
int day,month,year;
public:
void advance();
date(){
day=1;
month=1;
year=2018;
};
void setDate(){
cout<<day<<"/"<<month<<"/"<<year<<endl;
}
};
void date::advance(){
for(month=1;month=<12;month++){
for(day=1;day=<31;day++){
cout<<day<<"/"<<month<<"/"<<year<<endl;
}
}
}
int main(){
date d;
cout<<"Date set as:";
d.setDate();
cout<<"Setting the advance method"<<endl;
d.advance();
return 0;
}
It display In member function void date::setDate():
[Error] expected primary-expression before '<<' token
In member function 'void date::advance()':
[Error] expected primary-expression before '<<' token
I think you meant to write
cout<<day<<"/"<<month<<"/"<<year<<endl;
instead of
cout<<date<<"/"<<month<<"/"<<year<<endl;
in both the setDate and advance functions
I'm writing a c++ stack and queue implementation program, I finished the stack part, but when compiling I'm getting these errors
arrayListImp.cpp:18:19: error: expected unqualified-id
arrayList[++top]= x;
^
arrayListImp.cpp:28:13: error: 'arrayList' does not refer to a value
itemPoped=arrayList[top];
^
./arrayList.h:3:7: note: declared here
class arrayList{
^
arrayListImp.cpp:35:9: error: 'arrayList' does not refer to a value
return arrayList[top];
^
./arrayList.h:3:7: note: declared here
class arrayList{
^
arrayListImp.cpp:46:9: error: 'arrayList' does not refer to a value
cout<<arrayList[i]<<endl;
^
./arrayList.h:3:7: note: declared here
class arrayList{
^
4 errors generated.
Here is the header file
#ifndef ARRAYLIST_H
class arrayList{
public:
arrayList();
static const int maxSize = 10;
int array[10];
};
class stack : public arrayList{
public:
stack();
void push(int x);
void pop();
int Top();
int isEmpty();
void print();
int x;
int top;
int itemPoped;
int i;
};
#define ARRAYLIST_H
#endif
arrayListImp.cpp
#include <iostream>
#include "arrayList.h"
using namespace std;
//Stack implementation
stack::stack(){
top = -1;
}
void stack::push(int x){
if (top == maxSize -1){
cout<<"Stack overflow"<<endl;
}
else{
arrayList[++top]= x;
cout<<x<<", is pushed on to the stack"<<endl;
}
}
void stack::pop(){
if (top == -1){
cout<<"Stack underflow"<<endl;
}
else{
itemPoped=arrayList[top];
top--;
cout<<itemPoped<<", is poped from the stack"<<endl;
}
}
int stack::Top(){
return arrayList[top];
}
int stack::isEmpty(){
if (top == -1) return 1;
return 0;
}
void stack::print(){
cout<<"Stack: "<<endl;
for (i = 0; i<=top; i++){
cout<<arrayList[i]<<endl;
}
}
arrayListUse.cpp
#include <iostream>
#include "arrayList.h"
using namespace std;
int main()
{
//Stack testing
stack S;
S.push(1);S.print();
S.push(2);S.print();
S.push(3);S.print();
S.pop();S.print();
S.push(4);S.print();
//Queue testing
return 0;
}
Can you please point out to what I'm doing wrong here?
You should just read your error messages.
You should use array instead of arrayList, which is the name of the class. So just refer to the variable instead.
The error message you got is something like
test.cpp: In member function ‘void stack::push(int)’:
test.cpp:44:18: error: expected unqualified-id before ‘[’ token
arrayList[++top]= x;
^
When you check the line, you immediately see what is wrong there.
You declare a constructor arrayList::arrayList(), but you do not define it. Either you can drop the declaration, or you should implement it in the cpp-file.
arrayList::arrayList() {
// do some initialization
}
The error message you got is something like
/tmp/cc4y06YN.o:test.cpp:function stack::stack(): error: undefined reference to 'arrayList::arrayList()'
The code could compile, but it did not link. So all declarations may be correct, but a symbol was missing. This is usually the case when you declared something you referred to, but you never defined it.
You always have written
arrayList[...]
what is the name of your class but reading the code it seems like you wanted to write
array[...]
which would access the data.
I am new to c++, .I am trying to create a pgm that contains 2 classes ,out of which one class has a member function that would generate a callback function in another class though a function pointer, but i keep getting the following error.
#include <iostream>
#include <string>
using namespace std;
class B
{
private: std::string str1;
public: int generate_callback(std::string str1);
};
int B::generate_callback(std::string str1)
{
if ((str1=="Generate")||(str1=="generate"))
{
Cout<<"Callback generated ";
}
return 0;
}
class A : public B
{
public:
void count(int a,int b);
private: int a,b;
};
void A::count(int a, int b)
{
for ( a=1;a<b;a++){
if(a==50)
{
cout<<"Generating callback ";
goto exit;
}
exit: ;
}
}
int (*pt2function)(string)=NULL;
int main()
{
B obj1;
A obj2;
string str;
cout<<"To generate callback at int i=50 please enter 'generate'";
cin>>str;
obj2.count(1,100);
pt2function=&B::generate_callback;
(obj1.*pt2function)(str);
return 0;
}
The errors :
main.cpp:57: error: cannot convert 'int (B::*)(std::string) {aka int (B::*)(std::basic_string<char>)}' to 'int (*)(std::string) {aka int (*)(std::basic_string<char>)}' in assignment
pt2function=&B::generate_callback;
/home/adt/practice/N_practise/n_pract_2/pract2/main.cpp:58: error: 'pt2function' cannot be used as a member pointer, since it is of type 'int (*)(std::string) {aka int (*)(std::basic_string<char>)}'
(obj1.*pt2function)(str);
^
^
The variable pt2function is a pointer to a non-member function. Such a pointer is not compatible with a pointer to a member-function. Which is what the compiler tells you with the first error: A int (*)(string) is not compatible with a int (B::*)(string).
You need to define pt2function as a pointer to a B member function:
int (B::*pt2function)(string)=NULL;
Now you can initialize or assign a matching member function of B to the variable pt2function.
This also solves the second errors, which basically says that in your current code the variable pt2function is not a pointer to a member function, and therefore can not be used as such.
Pointers to functions and pointers to member functions are really different beasts.
You have mainly two options to get it working in your code:
Change this line:
int (*pt2function)(string)=NULL;
To this:
int (B::*pt2function)(string)=NULL;
That is defining pt2function as a pointer to a member function of B that gets a string and returns an int.
Declare the generate_callback as a static method and invoke it as pt2function(str); in your main function.
In fact, a static member function can be assigned to a pointer to function like the one you have already in use.
I'm getting this error on line 6:
error: expected unqualified-id before '{' token
I can't tell what's wrong.
#include <iostream>
using namespace std;
class WordGame;
{ // <== error is here on line 6
public:
void setWord( string word )
{
theWord = word;
}
string getWord()
{
return theWord;
}
void displayWord()
{
cout << "Your word is " << getWord() << endl;
}
private:
string theWord;
}
int main()
{
string aWord;
WordGame theGame;
cin >> aWord;
theGame.setWord(aWord);
theGame.displaymessage();
}
There should be no semicolon here:
class WordGame;
...but there should be one at the end of your class definition:
...
private:
string theWord;
}; // <-- Semicolon should be at the end of your class definition
As a side note, consider passing strings in setWord() as const references to avoid excess copying. Also, in displayWord, consider making this a const function to follow const-correctness.
void setWord(const std::string& word) {
theWord = word;
}
Get rid of the semicolon after WordGame.
You really should have discovered this problem when the class was a lot smaller. When you're writing code, you should be compiling about every time you add half a dozen lines.
Semicolon should be at the end of the class definition rather than after the name:
class WordGame
{
};
For what it's worth, I had the same problem but it wasn't because of an extra semicolon, it was because I'd forgotten a semicolon on the previous statement.
My situation was something like
mynamespace::MyObject otherObject
for (const auto& element: otherObject.myVector) {
// execute arbitrary code on element
//...
//...
}
From this code, my compiler kept telling me:
error: expected unqualified-id before for (const auto& element: otherObject.myVector) {
etc...
which I'd taken to mean I'd writtten the for loop wrong. Nope! I'd simply forgotten a ; after declaring otherObject.
For anyone with this situation: I saw this error when I accidentally used my_first_scope::my_second_scope::true in place of simply true, like this:
bool my_var = my_first_scope::my_second_scope::true;
instead of:
bool my_var = true;
This is because I had a macro which caused MY_MACRO(true) to expand into my_first_scope::my_second_scope::true, by mistake, and I was actually calling bool my_var = MY_MACRO(true);.
Here's a quick demo of this type of scoping error:
Program (you can run it online here: https://onlinegdb.com/BkhFBoqUw):
#include <iostream>
#include <cstdio>
namespace my_first_scope
{
namespace my_second_scope
{
} // namespace my_second_scope
} // namespace my_first_scope
int main()
{
printf("Hello World\n");
bool my_var = my_first_scope::my_second_scope::true;
std::cout << my_var << std::endl;
return 0;
}
Output (build error):
main.cpp: In function ‘int main()’:
main.cpp:27:52: error: expected unqualified-id before ‘true’
bool my_var = my_first_scope::my_second_scope::true;
^~~~
Notice the error: error: expected unqualified-id before ‘true’, and where the arrow under the error is pointing. Apparently the "unqualified-id" in my case is the double colon (::) scope operator I have just before true.
When I add in the macro and use it (run this new code here: https://onlinegdb.com/H1eevs58D):
#define MY_MACRO(input) my_first_scope::my_second_scope::input
...
bool my_var = MY_MACRO(true);
I get this new error instead:
main.cpp: In function ‘int main()’:
main.cpp:29:28: error: expected unqualified-id before ‘true’
bool my_var = MY_MACRO(true);
^
main.cpp:16:58: note: in definition of macro ‘MY_MACRO’
#define MY_MACRO(input) my_first_scope::my_second_scope::input
^~~~~
I got this error because I was not declaring a variable and was using it further .
Here is my code why I was getting it.
It was because I was not declaring a variable for size of my >vector.
Just replace
int n=arr.size();
Replace Here,
int sumSubarrayMins(vector<int>& arr) {
int = arr.size();
long long sum;
long long ans =0;
for(long i =0;i<n;i++){
sum =0;
long mini=INT_MAX;
for(long long j =i;j<n;j++){
mini=min(mini,arr[j]);
sum+=mini;
}
ans+=sum;
}
return ans;
}