CODEWITHSUNDEEP
c++stl
Konrad
Konrad

Reputation: 40947

STL Vector comparison issue

With a vector defined as std::vector<std::string>, Wondering why the following is valid:

if ( vecMetaData[0] != "Some string" ) 
{
    ...

But not this:

switch ( vecMetaData[1] )
{
    ...

Visual studio complains :

error C2450: switch expression of type 'std::basic_string<_Elem,_Traits,_Ax>' is illegal
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>,
1>            _Ax=std::allocator<char>
1>        ]
1>        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

Upvotes: 4

Views: 1183

Answers (6)

Reed Hedges
Reed Hedges

Reputation: 1650

If you just want to check each thing in the vector, you could use the for_each standard library function. Or if you want to act on a subset of possible values, use find_if to get iterators for the matching items and then use a loop or for_each to act on them.

Upvotes: 1

albert
albert

Reputation:

neither of those are probably what you want anyhow... since I assume you want to use the std::string::compare function to do a string comparison

Upvotes: 1

MSalters
MSalters

Reputation: 179779

The easiest alternative BTW is a std::map<std::string, boost::function> StringSwitch;

This lets you say StringSwitch["Some string"](arguments...)

Upvotes: 1

Pieter
Pieter

Reputation: 17705

switch() needs an integral type (like int, char, ...)

string is not an integral type, neither does string have an implicit conversion to an integral type, so it can't be used in a switch statement

Upvotes: 15

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 506847

It is valid because the first will call the operator!= of std::string, which will accept a const char* as an argument. That, however, doesn't mean that std::string also has an operator some_integral_type() that will return a integral expression which switch needs.

Using operators in C++ does not necassary invoke the builtin meaning. Your code, for example, doesn't compare pointer values. It might invoke a user defined (in this case, the one of std::string) operator function.

Upvotes: 3

schnaader
schnaader

Reputation: 49719

You can use switch only for basic datatypes (int, char etc.).

Upvotes: 2

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