Namespaces
Namespaces. How Shall I Name Thee?. Why Namespaces. by default, function, class, global constant and variable scope is global in large programming projects multiple people may give different constructs the same name, resulting in name collision .
Namespaces
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Namespaces How Shall I Name Thee?
Why Namespaces • by default, function, class, global constant and variable scope is global • in large programming projects multiple people may give different constructs the same name, resulting in name collision. • words like sort, find, count and print are tempting • name collision may result in • compile-time error - when header is included • linker error • namespaces– designed to sub-divide global scope and avoid name collision
Defining Namespaces • define a namespace block, declare constructs inside namespace myNamespace{ const int limt=5; void myFunc(int a){ ++a; } } • functions, classes, etc. (constructs with separate definitions) may be defined outside namespace myNamespace{ const int limt=5; void myFunc(int); } void myNamespace::myFunc(int a){ ++a; } thus structured, the namespace is non-executable and should go into a header file
Employing Namespaces several styles • always employ scope resolution operator (explicit statement of scope) for(int i=0; i < myNamespace::limit; ++i) myNamespace::myFunc(i); • import a name – specify that you are employing a specific name in this file using myNameSpace::limit; using myNameSpace::myFunc; for(int i=0; i < limit; ++i) myFunc(i); • import all names in a namespace using namespace myNamespace; for(int i=0; i < limit; ++i) myFunc(i); • do not do in headers – opens namespace for all source files including this header
Scope of Importing variables can be imported either into global scope or into a block • importing into the global scope – imported variables can be used anywhere using myNameSpace::limit; int main(void){ for(int i=0; i < limit; ++i) myFunc(i); } • importing into the scope of a function – imported variables can be used inside function only int main(void){ using myNameSpace::limit; for(int i=0; i < limit; ++i) myFunc(i); }
std • has an extensive set of names used in programs. Contains • cout, cin, endl, vector, string, etc. • three styles of employing std: • explicit scope resolution of all names • pro: safest • con: program code becomes less terse • employ using with specific names • pro: more terse • con: have to maintain name list at beginning of the file • employ using namespace std; • pro: simplest, tersest • con: unexpected names are imported: sort, find, count are in std;do not use in header files!
Review Questions • what is the scope of a function? global variable? global constant? • what is a namespace and why is it needed? • how to define a namespace? is namespace an executable statement? • how do you define a function declared in a namespace? • the three styles of employing namespaces are • explicit statement namespace • importing specific name • importing all names in a namespace how do you employ each style? what are the advantages/disadvantages of each style? • which style should be employed in a header • what happens when using is put inside/outside a function definition?
