what is the use of include in c++
Include directive In C and C++, the #include preprocessor directive causes the compiler to replace that line with the entire text of the contents of the named source file (if included in quotes: "") or named header (if included in angle brackets: <>);[1] note that a header need not be a source file.[2] Inclusion continues recursively on these included contents, up to an implementation-defined nesting limit. Headers need not have names corresponding to files: in C++ standard headers are typically identified with words, like "vector", hence #include <vector> while in C standard headers have identifiers in the form of filenames with a ".h" extension, as in #include <stdio.h>. A "source file" can be any file, with a name of any form, but is most commonly named with a ".h" extension and called a "header file" (sometimes ".hpp" or ".hh" to distinguish C++ headers), though files with .c, .cc, and .cpp extensions may also be included (particularly in the Single Compilation Unit technique), and sometimes other extensions are used.
Cost-plus contract A cost-reimbursement contract is appropriate when it is desirable to shift some risk of successful contract performance from the contractor to the buyer. It is most commonly used when the item purchased cannot be explicitly defined, as in research and development, or in cases where there is not enough data to accurately estimate the final cost.