Removing '#include <algorithm>' does not violate code

Question

This may be a very stupid question, but the book I'm reading instructed me to write a piece of code that uses algorithms to scramble and arrange elements in a vector. For this, the book tells me to use the library of algorithms from the main C ++ library. Ok, so far I understand this, but after writing the code, I wanted to see what would break if I removed this library from the top of my code, and it surprised me that everything was still working.

This is the code I'm talking about. When I remove the "#include algorithm" from the top of the code, nothing will break. How can it be? Shouldn't the random_shuffle part be interrupted if you are not using this library?

#include <iostream> #include <vector> #include <algorithm> #include <ctime> #include <cstdlib> using namespace std; int main() { vector<int>::const_iterator iter; cout << "Creating a list of scores."; vector<int> scores; scores.push_back(1500); scores.push_back(3500); scores.push_back(7500); cout << "\nHigh Scores:\n"; for (iter = scores.begin(); iter != scores.end(); ++iter) { cout << *iter << endl; } cout << "\nFinding a score."; int score; cout << "\nEnter a score to find: "; cin >> score; iter = find(scores.begin(), scores.end(), score); if (iter != scores.end()) { cout << "Score found.\n"; } else { cout << "Score not found.\n"; } cout << "\nRandomizing scores."; srand(static_cast<unsigned int>(time(0))); random_shuffle(scores.begin(), scores.end()); cout << "\nHigh Scores:\n"; for (iter = scores.begin(); iter != scores.end(); ++iter) { cout << *iter << endl; } cout << "\nSorting scores."; sort(scores.begin(), scores.end()); cout << "\nHigh Scores:\n"; for (iter = scores.begin(); iter != scores.end(); ++iter) { cout << *iter << endl; } system("pause"); return 0; } 

Answer 1

The reason it works is because it is included in the header, which you also included.

For example, a vector may have included algorithms in it. This is common because they often have a headline only.

However, you cannot rely on a specific implementation of the standard library to include every title. (for example, it can work with MSVC, and it can break with gcc stdlib +++).

For this reason, I highly recommend including what you use, regardless of where it will be compiled. --- Please note that this is slightly different from “what you are referencing,” because forward declarations for dots and links in headers can significantly improve build time.

Answer 2

The C ++ standard does not define which headers are included in each standard header. This means that, for example, <vector> can #include <algorithm> for implementing library A, but not in implementing library B. It can also change between different editions of the same library or on its different ports. glibC ++, for example, once carefully cleaned their inclusion hierarchies.

This puts us C ++ programmers in some inconvenience, since we must necessarily include the correct ones, although some standard headers have already done this; if we are lazy, we risk that the compilation breaks on other platforms, the system rises and falls.

General rule:

Include what is needed according to the definition of the header. But not more.

“But no more,” because compilation times may become too slow.

Answer 3

The code works because the vector contains an algorithm inside. To check what is included and what is not, you can generate a pre-processor output by passing the -E flag to the compiler.

Write a sample file: temp.C, which has only one line:

 #include <vector> 

Now, if we compile the file as g++ -E temp.C , you can see this algorithm in the output.

Answer 4

Breaks here

 janus@Zeus :~$ g++ test.cpp test.cpp: In function 'int main()': test.cpp:27:52: error: no matching function for call to 'find(std::vector<int>::iterator, std::vector<int>::iterator, int&)' test.cpp:27:52: note: candidate is: /usr/include/c++/4.6/bits/streambuf_iterator.h:359:5: note: template<class _CharT2> typename __gnu_cxx::__enable_if<std::__is_char<_CharT2>::__value, std::istreambuf_iterator<_CharT2, std::char_traits<_CharT> > >::__type std::find(std::istreambuf_iterator<_CharT2, std::char_traits<_CharT> >, std::istreambuf_iterator<_CharT2, std::char_traits<_CharT> >, const _CharT2&) test.cpp:39:48: error: 'random_shuffle' was not declared in this scope test.cpp:47:38: error: 'sort' was not declared in this scope janus@Zeus :~$ 

gcc 4.6.1

Answer 5

The code does not compile for me when I deleted the #include algorithm string. Do you run the risk of patronizing strongly, are you sure you recompiled?

My compiler output:

 a.cpp(28) : error C3861: 'find': identifier not found a.cpp(40) : error C3861: 'random_shuffle': identifier not found a.cpp(48) : error C3861: 'sort': identifier not found 

Answer 6

Unfortunately, standard headers are allowed to include other standard headers, so there is no guarantee that your code will break if you forget it.

Answer 7

The #include preprocessor directive does not “include” the library in the linking process, it simply instructs the precompiler to read the header file and include it in the source file, which it compiles as a whole.

However, the algorithm header file may be included in another include file.

Note. A specific function or method can be declared and defined in the header file, so binding is not required.

edit: Your code is well written and should work properly. I would advise you to use the predefined endl newline endl and include it at the end of the output sequence, and not at the beginning of the following:

 cout << "Creating a list of scores." << endl; cout << "High Scores:" << endl;