CODEWITHSUNDEEP
c++classvectorconstructor
eplictical
eplictical

Reputation: 647

std::vector ctor compiles outside of class, but not inside?

Why does the explicit vector (size_type n) form work outside of a class but not inside? This compiles:

#include <vector>

int main() {
    std::vector<int> vec_(3); // set capacity to 3
    return 0;
}

But not this:

#include <vector>

class C {
public:
    std::vector<int> vec_(3); // set capacity to 3
};

int main() {
    return 0;
}

g++ --std=c++0x -Wall -Wextra  -g a.cpp
a.cpp:5:27: error: expected identifier before numeric constant
a.cpp:5:27: error: expected ‘,’ or ‘...’ before numeric constant

Why? :(

Upvotes: 0

Views: 126

Answers (3)

gnobal
gnobal

Reputation: 18465

The correct way to do this would be:

class C {
public:
    C() : vec_(3) {} // set capacity to 3 in constructor initialization list
    std::vector<int> vec_;
};

Upvotes: 7

yzt
yzt

Reputation: 9113

Because that's not a valid syntax in C++. The correct way would be:

#include <vector>

class C {
public:
    std::vector<int> vec_;

public:
    // You add a constructor and initialize member data there:
    C () : vec_(3) {}
};

There are other ways to do it, but this is the most widely used and accessible one.

Upvotes: 2

crashmstr
crashmstr

Reputation: 28573

What you want is:

class C {
public:
    std::vector<int> vec_;
    C() : vec_(3) { }
};

This will control how vec_ is constructed when you construct a C object.

Upvotes: 4

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