using TMap=std::map; why it’s a wrong declaration?
and why such declaration is complicated?
My stab at the answer: because #define is used for identifiers, which std::map is, vs. types, of which int is…
No… as the lecture was told we can use using TMap instead of #define, but define is better in this case
Ah - likely due to needing an argument list for the class std::map, which can’t be provided at this level… Thus the #define being a better option.
on the contrary #define int32 int; also is a wrong declaration
in the end what is different between #define and using
“#define” is used for token substitutions (such as constants, variable names, etc.), whereas “using” is for namespace declarations & type aliasing.
“std::map” is a variable name - which is a token; int is a type and thus needs type aliasing.
Fine lines, but there you go.
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and do you know what is different between map and pair?
it has the same syntax pair<char,bool>
std::pair is a struct template that provides a way to store two heterogeneous objects as a single unit.
std::map is a sorted associative container that contains key-value pairs with unique keys.
(quotes taken from documentation)
First answer here answers your question. In short, map is a little more robust, however, if needs are simple, may be overkill.
C++ code passes through certain stages to become a binary (.e.g EXE on Windows).
preprocessor > compiler > linker > EXE
Now #define is a preprocessor directive which is a direct token substitution before the compiler begins
So your code
#define TMap std::map
is always working as the compiler won’t see TMap at all (would be substituted during pre-processing)
On the other hand, using = is a compile-time alias declaration and you have to play with the compiler rules, and as std::map is not a complete type (unlike e.g. std::string) but a name of a template (a not so easy topic in C++), you have to specify if the using = is a complete type or a template in itself. E.g. the correct way of doing using = in this case would be this:
// This works
using TMap = std::map<char, bool>;
// This will work too. (aliasing the whole template):
template<class K, class V>
using TMap = std::map<K, V>;
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thank you 