看文档有如下描述：
vector - C++ Referencehttp://www.cplusplus.com/reference/vector/vector/
(1) template
void emplace_back (Args&&... args);
Construct and insert element at the end
Inserts a new element at the end of the vector, right after its current last element. This new element is constructed in place using args as the arguments for its constructor.
This effectively increases the container size by one, which causes an automatic reallocation of the allocated storage space if -and only if- the new vector size surpasses the current vector capacity.
The element is constructed in-place by calling allocator_traits::construct with args forwarded.
A similar member function exists, push_back, which either copies or moves an existing object into the container.
(2) void push_back (const value_type& val);
Add element at the end
Adds a new element at the end of the vector, after its current last element. The content of val is copied (or moved) to the new element.
This effectively increases the container size by one, which causes an automatic reallocation of the allocated storage space if -and only if- the new vector size surpasses the current vector capacity.
实际感觉没太大区别，可能emplace_back的allocator_traits::construct没有充分理解和运用，总之存储对象时会创建对象和复制，并且容量增量会根据当前的size不同而不同。
emplace_back:
#include#include class Obj { public: Obj(int i) {this->i = i; std::cout << "constructor for " << i << "n";} ~Obj() {std::cout << "de-construct for " << i << "n";} Obj(const Obj &obj){if(&obj==this)return; i=obj.i; std::cout << "copy constructor for " << i << std::endl;} public: int i; }; int main() { std::cout << "--------------------------------n"; std::vector obj_vec1; int i=0; for(; i<4; i++) { Obj obj(i); obj_vec1.emplace_back(obj); std::cout << "capacity: " << obj_vec1.capacity() << std::endl; } //std::cout << "capacity: " << obj_vec1.capacity() << std::endl; std::cout << "-------------------------------n"; for(; i<8; i++) { Obj obj(i); obj_vec1.emplace_back(obj); std::cout << "capacity: " << obj_vec1.capacity() << std::endl; } //std::cout << "capacity: " << obj_vec1.capacity() << std::endl; std::cout << "-------------------------------n"; //obj_vec1.reserve(10); for(; i<12; i++) { Obj obj(i); obj_vec1.emplace_back(obj); std::cout << "capacity: " << obj_vec1.capacity() << std::endl; } //std::cout << "capacity: " << obj_vec1.capacity() << std::endl; std::cout << "-------------------------------n"; for(auto &v : obj_vec1) { std::cout << v.i << " "; } std::cout << std::endl; return 0; }
--------------------------------
constructor for 0
copy constructor for 0
capacity: 1
de-construct for 0
constructor for 1
copy constructor for 1
copy constructor for 0
de-construct for 0
capacity: 2
de-construct for 1
constructor for 2
copy constructor for 2
copy constructor for 0
copy constructor for 1
de-construct for 0
de-construct for 1
capacity: 4
de-construct for 2
constructor for 3
copy constructor for 3
capacity: 4
de-construct for 3
-------------------------------
constructor for 4
copy constructor for 4
copy constructor for 0
copy constructor for 1
copy constructor for 2
copy constructor for 3
de-construct for 0
de-construct for 1
de-construct for 2
de-construct for 3
capacity: 8
de-construct for 4
constructor for 5
copy constructor for 5
capacity: 8
de-construct for 5
constructor for 6
copy constructor for 6
capacity: 8
de-construct for 6
constructor for 7
copy constructor for 7
capacity: 8
de-construct for 7
-------------------------------
constructor for 8
copy constructor for 8
copy constructor for 0
copy constructor for 1
copy constructor for 2
copy constructor for 3
copy constructor for 4
copy constructor for 5
copy constructor for 6
copy constructor for 7
de-construct for 0
de-construct for 1
de-construct for 2
de-construct for 3
de-construct for 4
de-construct for 5
de-construct for 6
de-construct for 7
capacity: 16
de-construct for 8
constructor for 9
copy constructor for 9
capacity: 16
de-construct for 9
constructor for 10
copy constructor for 10
capacity: 16
de-construct for 10
constructor for 11
copy constructor for 11
capacity: 16
de-construct for 11
-------------------------------
0 1 2 3 4 5 6 7 8 9 10 11
de-construct for 0
de-construct for 1
de-construct for 2
de-construct for 3
de-construct for 4
de-construct for 5
de-construct for 6
de-construct for 7
de-construct for 8
de-construct for 9
de-construct for 10
de-construct for 11
push_back:
#include#include class Obj { public: Obj(int i) {this->i = i; std::cout << "constructor for " << i << "n";} ~Obj() {std::cout << "de-construct for " << i << "n";} Obj(const Obj &obj){if(&obj==this)return; i=obj.i; std::cout << "copy constructor for " << i << std::endl;} public: int i; }; int main() { std::cout << "--------------------------------n"; std::vector obj_vec1; int i=0; for(; i<4; i++) { Obj obj(i); obj_vec1.push_back(obj); std::cout << "capacity: " << obj_vec1.capacity() << std::endl; } //std::cout << "capacity: " << obj_vec1.capacity() << std::endl; std::cout << "-------------------------------n"; for(; i<8; i++) { Obj obj(i); obj_vec1.push_back(obj); std::cout << "capacity: " << obj_vec1.capacity() << std::endl; } //std::cout << "capacity: " << obj_vec1.capacity() << std::endl; std::cout << "-------------------------------n"; //obj_vec1.reserve(10); for(; i<12; i++) { Obj obj(i); obj_vec1.push_back(obj); std::cout << "capacity: " << obj_vec1.capacity() << std::endl; } //std::cout << "capacity: " << obj_vec1.capacity() << std::endl; std::cout << "-------------------------------n"; for(auto &v : obj_vec1) { std::cout << v.i << " "; } std::cout << std::endl; return 0; }
constructor for 0
copy constructor for 0
capacity: 1
de-construct for 0
constructor for 1
copy constructor for 1
copy constructor for 0
de-construct for 0
capacity: 2
de-construct for 1
constructor for 2
copy constructor for 2
copy constructor for 0
copy constructor for 1
de-construct for 0
de-construct for 1
capacity: 4
de-construct for 2
constructor for 3
copy constructor for 3
capacity: 4
de-construct for 3
-------------------------------
constructor for 4
copy constructor for 4
copy constructor for 0
copy constructor for 1
copy constructor for 2
copy constructor for 3
de-construct for 0
de-construct for 1
de-construct for 2
de-construct for 3
capacity: 8
de-construct for 4
constructor for 5
copy constructor for 5
capacity: 8
de-construct for 5
constructor for 6
copy constructor for 6
capacity: 8
de-construct for 6
constructor for 7
copy constructor for 7
capacity: 8
de-construct for 7
-------------------------------
constructor for 8
copy constructor for 8
copy constructor for 0
copy constructor for 1
copy constructor for 2
copy constructor for 3
copy constructor for 4
copy constructor for 5
copy constructor for 6
copy constructor for 7
de-construct for 0
de-construct for 1
de-construct for 2
de-construct for 3
de-construct for 4
de-construct for 5
de-construct for 6
de-construct for 7
capacity: 16
de-construct for 8
constructor for 9
copy constructor for 9
capacity: 16
de-construct for 9
constructor for 10
copy constructor for 10
capacity: 16
de-construct for 10
constructor for 11
copy constructor for 11
capacity: 16
de-construct for 11
-------------------------------
0 1 2 3 4 5 6 7 8 9 10 11
de-construct for 0
de-construct for 1
de-construct for 2
de-construct for 3
de-construct for 4
de-construct for 5
de-construct for 6
de-construct for 7
de-construct for 8
de-construct for 9
de-construct for 10
de-construct for 11
可以到这里验证一下: C++ Shellhttp://cpp.sh/
