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As an example, here is a program that copies a file named in to a file named out:
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int main() { ifstream infile("in"); ofstream outfile("out"); string s; while (getline(infile, s)) outfile << s << endl; return 0; }
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This program takes advantage of the fact that a string literal is effectively a pointer to the initial character of a null-terminated array If we don't want to have to give the name of the file as a literal, the best alternative is to store the file name in a string and then use the c_str member function that we will describe in 126/224 So, for example, if file is a string variable that contains the name of a file that we want to read, we can create an ifstream object that will read it by defining it as
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ifstream infile(filec_str());
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As a final example, here is a program that produces, on its standard output, a copy of the contents of all the files whose names are given as arguments to main:
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int main(int argc, char **argv) { int fail_count = 0; // for each file in the input list for (int i = 1; i < argc; ++i) { ifstream in(argv[i]); // if it exists, write its contents, otherwise generate an error message if (in) { string s; while (getline(in, s)) cout << s << endl; } else { cerr << "cannot open file " << argv[i] << endl; ++fail_count; } } return fail_count; }
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For each argument given to main ( 104/179), the program creates an ifstream object to read the file by that name If the object appears false when used as a condition, that means that the file does not exist, or that it cannot be read for some reason Accordingly, the program complains on cerr, and keeps a count of how many failures it had If the program created the ifstream object successfully, it reads the file, one line at a time, into s, and writes the contents of each line on the standard output When the program returns control to the system, it passes back the number of files that it was unable to read As usual, a return value of zero indicates success, which in this case will indicate that we were able to read all the files
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106 Three kinds of memory management
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So far, we have seen two distinct kinds of memory management, although we have not discussed them explicitly The first kind is usually called automatic memory management, and is associated with local variables: A local variable occupies memory that the system allocates when it encounters the variable's definition during execution The system automatically deallocates that memory at the end of the block that contains the definition Once a variable has been deallocated, any pointers to it become invalid It is the programmer's responsibility to avoid using such invalid pointers For example,
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// this function deliberately yields an invalid pointer // it is intended as a negative example don't do this! int* invalid_pointer() { int x; return &x; // instant disaster! }
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This function returns the address of the local variable x Unfortunately, when the function returns, doing so ends execution of the block that contains the definition of x, which deallocates x The pointer that &x created is now invalid, but the function tries to return it anyway What happens at this point is anybody's guess In particular, C++ implementations are not required to diagnose the error you get what you get If we want to return the address of a variable such as x, one way to do so is to use the other kind of memory management, by asking for x to be statically allocated:
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// This function is completely legitimate int* pointer_to_static() { static int x; return &x; }
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By saying that x is static, we are saying that we want to allocate it once, and only once, at some point before the first time that pointer_to_static is ever called, and that we do not want to deallocate it as long as our program runs There is nothing wrong with returning the address of a static variable; the pointer will be valid as long as the program runs, and it will be irrelevant afterward However, static allocation has the potential disadvantage that every call to pointer_to_static will return a pointer to the same object! Suppose we want to define a function such that each time we call it, we get a pointer to a brand new object, which stays around until we decide that we no longer want it To do so, we use dynamic allocation, which we request by using the new and delete keywords
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