debug template for array C++

Question

I want to create debug template for array for printing array. This is my template

#define debug(x) cerr << #x <<" = "; print(x); cerr << endl;

void print(ll t) {cerr << t;}
void print(int t) {cerr << t;}
void print(float t) {cerr << t;}
void print(string t) {cerr << t;}
void print(char t) {cerr << t;}

template <class T> void print(vector <T> v) {cerr << "[ "; for (T i : v) {print(i); cerr << " ";} cerr << "]";}

template <class T> void print(T arr[]) {cerr << "[ "; for (T i : arr) {print(i); cerr << " ";} cerr << "]";}

But its giving some errors for array but vector works fine,

 template <class T> void print(T arr[]) {cerr << "[ "; for (T i : arr) {print(i); cerr << " ";} cerr << "]";}
                                                       ^~~
A.cpp:16:55: note: suggested alternatives:
In file included from c:\mingw\lib\gcc\mingw32\6.3.0\include\c++\mingw32\bits\stdc++.h:95:0,
                 from A.cpp:1:
c:\mingw\lib\gcc\mingw32\6.3.0\include\c++\valarray:1206:5: note:   'std::begin'
     begin(const valarray<_Tp>& __va)
     ^~~~~
c:\mingw\lib\gcc\mingw32\6.3.0\include\c++\valarray:1206:5: note:   'std::begin'
A.cpp:16:55: error: 'end' was not declared in this scope
 template <class T> void print(T arr[]) {cerr << "[ "; for (T i : arr) {print(i); cerr << " ";} cerr << "]";}
                                                       ^~~
A.cpp:16:55: note: suggested alternatives:
In file included from c:\mingw\lib\gcc\mingw32\6.3.0\include\c++\mingw32\bits\stdc++.h:95:0,
                 from A.cpp:1:
c:\mingw\lib\gcc\mingw32\6.3.0\include\c++\valarray:1226:5: note:   'std::end'
     end(const valarray<_Tp>& __va)
     ^~~
c:\mingw\lib\gcc\mingw32\6.3.0\include\c++\valarray:1226:5: note:   'std::end'

So how to create a debug for array ?

You might want to check your const correctness of your code, printing should not be able to modify data. Also use const T& instead of T as argument types (to avoid unnecessary copies). — Pepijn Kramer, Jun 27 '22 at 06:11
the syntax of c and c++ are almost same so I thought that people who know c can also be able to understand this query and provide a solution if possible @Yunnosch — Pranav Patil, Jun 27 '22 at 06:29
i think there is quite a good enough explanation for this question and all the code giving is necessary to explain question @n. 1.8e9-where's-my-share m. — Pranav Patil, Jun 27 '22 at 06:31

score 3 · Accepted Answer · answered Jun 27 '22 at 06:23

A range-based for loop (for(T i : v) internally uses std::begin and std::end to determine the range it hast to iterate over. These methods work for arrays whose size is fixed at compile time, e.g. int arr[10];, for any standard container, like std::vector, std::list, or any class that has a .begin() and .end()-method that return iterators, I think.

However, it does not work for pointers.

In fact print(T arr[]) equals print(T *arr). You can't determine the length of an array, when you only got a pointer. You can't even determine wether or not this pointer is pointing to an array. So you can't use range-based for loops with pointers.

So how to create a debug for array ?

If you want to use dynamically allocated arrays you have to pass the arrays size along

template <class T> void print(T v[], size_t size)

and use a normal for loop

for(size_t i = 0; i < size; i++) { ... }

Another possibility is to make a template for array-size, as in this question: Can someone explain this template code that gives me the size of an array?

debug template for array C++

1 Answers1