C++ 標準ライブラリを利用して、より簡潔なコードを作成できます。確かに、一度にすべてを学ぶのは簡単ではありません (練習が必要です。これが、私がこのような質問に答える理由の 1 つです)。
次のコードは、あなたが求めていることを実行します。つまり、ファイルから読み取り、内容を画面に出力します。より柔軟にすることができます (たとえば、コマンド ライン引数を介してファイル名を指定する)。例外を処理したり、ファイルを閉じたり (この場合、プログラムの終了時に自動的に行われます) などに拡張する必要があります。
各コンストラクトの役割を知りたい場合は、cppreference.com にアクセスしてください。とても役に立ちました。
// These statements "bring into the program" the "hooks" needed to
// invoque the functionality available in the Standard Library. In C++
// you "only pay for what you use", that is why you should only
// "include" what you need.
// header name it providest the means to ...
#include<iterator> // "walk through" a thing
#include<iostream> // handle input (i) / output (o) streams
#include<fstream> // handle file streams
#include<vector> // handle collections of items
// core entry point to the program
int main() {
// all data will be read into a collection of integers (in this
// case, you could have created a collection of any type). Vectors
// can grow and shrink automatically, and can be traversed
// (iterated; walked-through) efficiently. There are other
// "collections" available (list, array, set, queue, stack, etc)
// each with different capabilities
std::vector<int> data;
// create an "input stream" feed from a file called "numbers.txt"
std::ifstream fp("numbers.txt");
// copy the contents of the "input stream" to the vector. An
// "istream_iterator" is simply an "agent" which can "walk through"
// any "input stream". In this case, "walk through" means "give me
// integers until there are none". A "back inserter" is an "agent"
// which can "push back" stuff into a container. In this case it
// will "push back" integers into a vector; the vector being "data",
// our container defined above.
std::copy(std::istream_iterator<int>(fp), // this denotes "start of stream"
std::istream_iterator<int>(), // this denodes "end of stream"
std::back_inserter<std::vector<int>>(data)); // this is
// where I
// want to
// store the
// copied
// value
// now that I have copied all the values of the file into the data
// vector, I want to copy them again, this time to the screen output
// (called "cout"). An "ostream iterator" can "walk through"
// anything that is an "output stream".
std::copy(data.begin(), // denotes "start of data"
data.end(), // denotes "end of data"
std::ostream_iterator<int>(std::cout, " "));
// finally --- notice that we do not need the vector at all! We
// could have copied the input stream directly into the output
// stream! The statement would be as follows:
fp.seekg(0); // this tells the "stream" to "go back to the
// beginning". We need to do this because I already
// "walked through it", and it is currently at the end.
std::copy(std::istream_iterator<int>(fp), // this denotes "start of stream"
std::istream_iterator<int>(), // this denodes "end of stream"
std::ostream_iterator<int>(std::cout, " "));
return 0;
}
コンパイル命令 (gcc 4.8.1 を使用):g++ example.cpp -std=c++11
出力：1 2 3 4 5 5 4 3 2 1 1 2 3 4 5 5 4 3 2 1 1 2 3 4 5
では、ベクトルを使わずにやってみませんか?
#include<iterator>
#include<iostream>
#include<fstream>
int main() {
// We can copy the input stream directly into the output stream!
std::ifstream fp("numbers.txt");
std::copy(std::istream_iterator<int>(fp),
std::istream_iterator<int>(),
std::ostream_iterator<int>(std::cout, " "));
return 0;
}
出力：1 2 3 4 5 5 4 3 2 1 1 2 3 4 5 5 4 3 2 1 1 2 3 4 5